(CNN) -- Thousands gathered outside Warsaw's presidential palace on Saturday evening to lay flowers and light candles in honor of Polish President Lech Kaczynski, who was killed in a plane crash in western Russia earlier in the day.
Kaczynski's wife and several top military officials were also killed in the crash.
"There is going to be a huge gap in public life in Poland," said resident Magdalena Hendrysiak. "The most important people are dead."
Russian officials said that 97 people were killed when the plane carrying Kaczynski crashed as it was trying to land at an airport near the city of Smolensk.
Parliament Speaker Bronislaw Komorowski took over as acting president and declared it "a time for national mourning."
Kaczynski, 60, had been traveling with a Polish delegation to Russia for the 70th anniversary of the Russian massacre of Polish prisoners of war in the village of Katyn. Some 20,000 Poles, including soldiers and civilians, were executed there during World War II.
The event was to be a milestone in the easing of relations between Russia and Poland, strained for decades over Poland's close relationship to the United States and because of lingering ill will over Russia's campaign against the Poles in the 1940s.
"A shared tragedy of this sort could give a boost to further improvement in Polish-Russian relations, which ... were on the mend," said Tomas Valasek of the Center for European Reform. "This tragedy might accelerate that trend."
Polish Prime Minister Donald Tusk and Russian Prime Minister Vladimir Putin appeared together in Smolensk on Saturday night.
Russia has declared Monday a day of mourning, though Russian television portrayed Kaczynski in a largely negative light on Saturday, highlighting the Polish president's criticisms of Moscow.
The Polish military plane carrying Kaczynski originated in Warsaw, the Polish Defense Ministry said.
It was just a few miles east of Katyn when it crashed around 10:50 a.m. (2:50 a.m. ET) on the outskirts of the town of Pechorsk, close to Smolensk, the Investigation Committee of the Russian prosecutor's office said.
Russian officials on Saturday emphasized that there is no evidence it was responsible for the crash.
The Investigation Committee said the plane, a Tupolev-154, was trying to land in heavy fog. A Russian military official said that air traffic control in Smolensk had tried to divert the plane to another airport because of inclement weather.
"The air traffic control officer gave several orders to divert but the plane continued with its descent," said Alexander Aleshin, a top Russian Air Force official. "Unfortunately, this ended in tragedy."
Russian President Dmitry Medvedev appointed Putin to head an inquiry commission and sent Emergency Situations Minister Sergei Shoigu to the scene of the crash, the Kremlin said.
"We had a lot in common as presidents and as ordinary people and I can assure you that the death of Lech Kaczynski will be investigated and that we will uncover all the circumstances of this plane crash," Medvedev said Saturday.
Other Polish officials killed in the crash include Aleksander Szczyglo, the head of the National Security Office; Jerzy Szmajdzinski, the deputy parliament speaker; Andrzej Kremer, the deputy foreign minister; and Gen. Franciszek Gagor, the army chief of staff, according to Kaczynski's Law and Justice Party.
The party also said that Slawomir Skrzypek, head of the National Bank of Poland, was killed.
"The entire top military brass, including the chief of defense and all the services, were on the plane," Valasek said. "You're looking at a situation, in effect, of the decapitation of the military services."
Putin spoke Saturday at the crash site, where charred pieces of the airplane were strewn through a wooded area. Some pieces, including one of the wheel wells, were upside down.
"As our first priority, we must establish the causes of this tragedy," he said. "As a second priority, we must do everything in our power to assist the families and relatives of the deceased."
The plane carrying Kaczynski was refurbished and repaired last year, according to Alexei Gusev, general director of Aviakor Factory, the company that performed the service.
"Speaking openly, we believe that this tragedy could not have been caused by equipment failure," he said.
Kaczynski had been president since December 2005, after he defeated rival Tusk in the second round of voting.
The two men did not have a good relationship. In parliamentary elections in 2007, Tusk's Civic Platform beat the Law and Justice Party of Kaczynski's twin brother, Jaroslaw Kaczynski, who was prime minister at the time.
Tusk put political differences aside as he spoke to the nation Saturday.
"I wanted, in the name of all the Polish people, to pass words of condolences to the familiy of the Polish president, to his daughter, to his mother, to his brother, and to all the families of all the victims," he said.
Tusk and Polish Cabinet ministers held a special meeting Saturday morning to discuss the situation.
Elections must now be held within 60 days, said Dariusz Rosati, Poland's former foreign minister.
Valasek pointed out, however, that the Polish president is the head of state, not head of government -- meaning essential services will continue.
"The role of the Polish president is not quite ceremonial ... he has some very real powers, but at the end of the day, the day-to-day running of the government is in the hands of the prime minister and the (cabinet) ministers," Valasek said. "Continuity is assured in ways that would not necessarily be assured in the case of the death of the U.S. president."
Meanwhile Saturday, condolences poured in from around the world, including from British Prime Minister Gordon Brown, German Chancellor Angela Merkel, and Britain's Queen Elizabeth and Prince Charles.
"President Kaczynski was a distinguished statesman who played a key role in the Solidarity movement, and he was widely admired in the United States as a leader dedicated to advancing freedom and human dignity," U.S. President Barack Obama said in a statement.
CNN's Melissa Gray in London, England, and Max Tkachenko in Bishkek, Kyrgyzstan, contributed to this report.