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London (CNN) -- UK and world leaders led tributes to Britain's "Iron Lady," former Prime Minister Margaret Thatcher on Monday, with words reflecting on her passion, courage and determination -- as well as her polarizing politics.
"Margaret Thatcher took a country that was on its knees and made Britain stand tall again," Prime Minister David Cameron said outside 10 Downing Street.
"We cannot deny that Margaret Thatcher divided opinion. For many of us, she was and is an inspiration. For others, she was a force to be defined against."
But if one thing runs though everything she did, he said, it was "her lion-hearted love of this country. She was the patriot prime minister and she fought for Britain's interests every step of the way."
Cameron cut short a trip to Spain to return to London following news of her death.
Buckingham Palace said the "Queen was sad to hear the news of the death of Baroness Thatcher. Her majesty will be sending a private message of sympathy to the family."
U.S. President Barack Obama said the world had "lost one of the great champions of freedom and liberty" and the United States had lost "a true friend."
"As a grocer's daughter who rose to become Britain's first female prime minister, she stands as an example to our daughters that there is no glass ceiling that can't be shattered," Obama said in a White House statement.
"As prime minister, she helped restore the confidence and pride that has always been the hallmark of Britain at its best. And as an unapologetic supporter of our transatlantic alliance, she knew that with strength and resolve we could win the Cold War and extend freedom's promise."
U.S. Secretary of State John Kerry said the world had "lost a transformative leader who broke the glass ceiling in global politics" and "defined grit on the world stage."
Thatcher took the helm amid tumultuous times, he said. "She would face wars abroad, terrorism at home, and deep uncertainty about the United Kingdom's future. She met all these challenges and many others with unyielding drive and courage."
British Foreign Secretary William Hague told CNN that Thatcher had taught her country about political determination and leadership as Conservative Party leader and prime minister.
"She stuck to things through thick and thin, through many demands that she should change course," he said. "She really showed what difference an individual could make, what difference a leader could make."
At home, she "changed the entire political battleground," he said, with her influence still felt by all Britain's parties today. "She was at times, of course, polarizing, but no leader could have accomplished what she accomplished purely based on consensus decisions."
Thatcher had also shown people in Britain and around the world the value of freedom, democracy and human rights, he said.
Former Prime Minister John Major, who served in Thatcher's Cabinet and took the reins when a party rebellion over Europe and an unpopular tax forced her from office in 1990, said hers was "a very remarkable life."
When she broke the glass ceiling in politics in 1979, she did something no one could even have dreamed of at the start of the decade, he told CNN. "She was the right prime minister for the right time," he said.
As for the vitriol directed toward her by her critics, "in private she did care. She did understand it, and she did care, but she had a mission and she wasn't going to be put off that mission."
He had "very big shoes to fill" when he followed Thatcher into 10 Downing Street, Major said.
Former British Prime Minister Tony Blair, who led the opposition Labour Party in government from 1997 to 2007, said Thatcher was a "towering political figure" who would be greatly missed.
"Very few leaders get to change not only the political landscape of their country but of the world. Margaret was such a leader. Her global impact was vast," he said in a statement.
Some of the changes she made in Britain were retained to a degree by his government, he said, and their influence spread around the world.
"Even if you disagreed with her as I did on certain issues and occasionally strongly, you could not disrespect her character or her contribution to Britain's national life," he said.
'A great lady'
Former U.S. Secretary of State Henry Kissinger told CNN that Thatcher was a "tremendous prime minister. She was a great lady, she had very strong opinions. And to those of us who knew her over the decades, she was a very warm person, which is not the public image that is often given."
Thatcher's great achievement for Britain was its success in the Falklands crisis, he said, referring to the war over the disputed islands known to Argentina, which also claims them, as Las Malvinas.
"For the United States, it was her staunch loyalty and commitment to the Atlantic alliance -- she was a reliable and steady ally."
She was also one of the first leaders to see the way forward to ending the Cold War, he said, spotting the potential for a new kind of leader in Mikhail Gorbachev.
"I like Mr. Gorbachev. We can do business together," she said in December 1984, three months before he became Soviet leader.
Her dealings with former U.S. President Ronald Reagan, a fellow political conservative, were notably warm, Kissinger said.
"Margaret Thatcher, in her relationship with Ronald Reagan, gave it an additional personal dimension and it was unusually close, and they acted between themselves almost as if they were part of the same government."
Ending the Cold War
Gorbachev, too, paid tribute to a "great politician" with a strong voice, who helped shape 20th century history.
"Our first meeting, in 1984, marked the beginning of a relationship that was at times difficult, not always smooth, but on both sides serious and responsible," he said.
"Gradually, our relationship became more and more friendly. At the end, we were able to reach an understanding, and it was a contribution to the change in atmosphere between our country and the West and to the end of the Cold War."
Former U.S. President George H.W. Bush -- whose first years in the White House overlapped with the end of Thatcher's time as prime minister but who served as vice president at the height of her power and influence -- called her one of the "fiercest advocates of freedom and free markets."
He said in a statement that Thatcher was a leader of rare character who "carried high the banner of her convictions, and whose principles in the end helped shape a better, freer world."
Thatcher famously told Bush, "this is no time to go wobbly," as the first Gulf War loomed.
His son, former President George W. Bush, said that he and his wife, Laura, were grieving the loss of a "strong woman and friend" in Thatcher.
Reagan's widow, Nancy, said she was terribly saddened by the loss of a dear and trusted friend.
"Ronnie and Margaret were political soul mates, committed to freedom and resolved to end Communism.
"As prime minister, Margaret had the clear vision and strong determination to stand up for her beliefs at a time when so many were afraid to 'rock the boat.' As a result, she helped to bring about the collapse of the Soviet Union and the liberation of millions of people."
Former President Bill Clinton said Thatcher "understood that the special relationship which has long united our two nations is an indispensable foundation for peace and prosperity.
"Our strong partnership today is part of her legacy."
'Great hurt to Irish and British people'
But Northern Ireland politician and Sinn Fein leader Gerry Adams gave a very different view of Thatcher's legacy.
"Margaret Thatcher did great hurt to the Irish and British people during her time as British prime minister," he said in a statement.
"Working class communities were devastated in Britain because of her policies.
"Her role in international affairs was equally belligerent whether in support of the Chilean dictator (Agusto) Pinochet, her opposition to sanctions against apartheid South Africa; and her support for the Khmer Rouge.
"Here in Ireland, her espousal of old draconian militaristic policies prolonged the war and caused great suffering."
She will be remembered in particular for "her shameful role during the epic hunger strikes of 1980 and 81" and her Irish policy "failed miserably," he concluded.
His longtime political opponent in Northern Ireland, the former Rev. Ian Paisley, now Lord Bannside, had kinder words for Thatcher.
"In every phase of life she was great -- great as a woman, great as a wife, great as a mother, great as a political candidate, great as a Member of Parliament, especially as the first woman prime minister, great as a winner of the war, and great as a member of the House of Lords," he said.
Irish Prime Minister Enda Kenny also gave a more positive assessment.
"While her period of office came at a challenging time for British-Irish relations, when the violent conflict in Northern Ireland was at its peak, Mrs Thatcher signed the Anglo-Irish Agreement which laid the foundation for improved North-South cooperation and ultimately the Good Friday Agreement."
Falklands: 'We will always be thankful'
Officials in the Falkland Islands said they would never forget Thatcher's decision to defend the South Atlantic territory in 1982.
"Her friendship and support will be sorely missed, and we will always be thankful for all that she did for us," said Mike Summers of the Falkland Islands Legislative Assembly.
The United Kingdom and Argentina went to war over the territory in 1982 after the then-military government in Argentina landed troops on the islands. Argentina put its death toll from the conflict at around 645. Britain says its civil and military losses amounted to 255.
For more than a year, renewed rhetoric between the two countries over the islands has escalated to a fever pitch, and Argentina's state-run Telam news agency offered an unflinching look Monday at the South American country's take on Thatcher's legacy.
Articles described her as "a symbol of war," "an expression of inequality" and "a great destroyer."
Britain in Europe
European leaders paid tribute to Thatcher's role in shaping Britain's place in Europe.
European Commission President Jose Manuel Barroso said: "She was without doubt a great stateswoman ... and a circumspect yet engaged player in the European Union. She will be remembered for both her contributions to and her reserves about our common project.
"She signed the Single European Act and helped bring about the Single Market. She was a leading player in bringing into the European family the Central and Eastern European countries which were formerly behind the Iron Curtain."
German Foreign Minister Guido Westerwelle said Thatcher "changed Britain permanently. In the same way, she leaves European and world history a great legacy.
"Margaret Thatcher counted among the very few people who you knew, even within their lifetime, had helped write history. She always had her own unmistakeable, pithy and distinctive opinion."
French President Francois Hollande said that Thatcher's relationship with France "was always frank and honest."
She had established "a constructive and fruitful dialogue" with former President Francois Mitterrand and worked to strengthen the ties between Britain and France, he added.
Herman Van Rompuy, president of the European Council, said Thatcher "was a remarkable personality and one of the most influential European leaders of her time."
U.N. Secretary General Ban Ki-moon said the world owed a great deal to Thatcher's leadership.
"She was a pioneering leader for her contribution to peace and security, particularly at the height of the Cold War," he said.
"She was also a great model as the first woman prime minister of the United Kingdom, who not only demonstrated her leadership but has given such great hope for many women for equality, gender equality in Parliament."
NATO Secretary-General Anders Fogh Rasmussen said Thatcher "was an extraordinary politician who was a staunch defender of freedom, a powerful advocate of NATO and the transatlantic bond."
Israeli Prime Minister Benjamin Netanyahu said Thatcher had inspired a generation of political leaders.
"She was truly a great leader, a woman of principle, of determination, of conviction, of strength; a woman of greatness," he said. "She was a staunch friend of Israel and the Jewish people."
Canadian Prime Minister Stephen Harper said the world had lost a "giant among leaders" with Thatcher's passing.
She "had that rarest of abilities to herself personify and define the age in which she served. Indeed, with the success of her economic policies, she defined contemporary conservatism itself," he said.
Nigeria's President Goodluck Jonathan said she would be remembered "for her very unique, distinctive and purposeful leadership which restored pride and respect to her country" and put it back on the global stage.
'Love her or hate her'
Hollywood star Meryl Streep's portrayal of Thatcher in the 2011 film "The Iron Lady" gave some younger viewers their first insight into the former politician's character.
"To me she was a figure of awe for her personal strength and grit," Streep said Monday.
"To have withstood the special hatred and ridicule, unprecedented in my opinion, leveled in our time at a public figure who was not a mass murderer; and to have managed to keep her convictions attached to fervent ideals and ideas -- wrongheaded or misguided as we might see them now -- without corruption -- I see that as evidence of some kind of greatness, worthy for the argument of history to settle."
Iain Dale, author of the biography "Margaret Thatcher: In Her Own Words," told CNN the politician was unique in the way that her legacy still dominates British politics long after she left office.
And she polarized the country because she was a politician of conviction, he said.
"You see the reaction on Twitter to her death. You have people who absolutely idolized her who are devastated today, and you have others who are spewing out the most hateful things," he said.
"That's what happens when you are a conviction politician -- people either love you or hate you."
Thatcher's unbending opposition to the trade unions is behind many people's dislike, he said.
But, Dale said, she transformed the British economy, pulling it out of a malaise and restoring the nation's sense of self-belief.
When he last met her, about three years ago, she was still sharp when it came to remembering details of her time in office, Dale said, although her dementia affected her grasp of what was happening in the present.
Her electoral success can be credited to her ability to tap into the national psyche, he said, in the same way that Tony Blair managed in the 1990s.
"She understood what drove 'middle Britain,'" he said. "She was both a conviction politician and a popularist -- and that kind of politician is usually quite popular."
The current speaker of the UK House of Commons, John Bercow, said he was greatly saddened by her death.
As a 16-year-old student in Thatcher's own constituency at the time of the 1979 general election which put her in power, he is part of a generation of British politicians to have grown up in her shadow.
Parliament will come back early from recess to allow lawmakers to pay their tributes to Thatcher at a special session on Wednesday, Cameron said.
Mayor of London Boris Johnson said on Twitter: "Her memory will live long after the world has forgotten the grey suits of today's politics."
CNN's Roger Clark, Per Nyberg, Jonathan Wald, Richard Allen Greene, Laura Perez Maestro, Paula Newton, Devon Sayers, Natasha Maguder, Kim Norgaard, Nana Karikari-apau, Karen Zuker, Kay Jones, Maria Ebrahimji, Catherine E. Shoichet and journalist Peter Taggart contributed to this report.