Warsaw, Poland (CNN) -- Numerous world leaders were forced to cancel plans to travel to Poland for the funeral of President Lech Kaczynski on Sunday, as volcanic ash from Iceland continued to ground flights to and from Europe.
French President Nicolas Sarkozy, German Chancellor Angela Merkel and U.S. President Barack Obama were among the heads of state who abandoned plans to travel to Poland for the funeral.
Contrary to earlier reports that Sarkozy was already in Poland, his Web site said Saturday that "given the extreme weather that paralyzed the air traffic over most of Europe, (the) President of the Republic will not go to Krakow (Poland) to attend the funeral of President Kaczynski and his wife, as originally planned."
Kaczynski, his wife, dignitaries and top military leaders were killed April 10 when their plane crashed in bad weather in western Russia. They were on their way to a service commemorating Polish prisoners of war massacred in Russia during World War II.
Merkel's Web site said her cancellation came "with the greatest expression of regret," and that the Polish government had been notified. Polish Foreign Minister Radoslaw Sikorski "expressed full understanding," the site said.
The White House said the United States would be represented at the funeral by Lee Feinstein, U.S. ambassador to Poland.
"I spoke with acting President (Bronislaw) Komorowski and told him that I regret that I will not be able to make it to Poland due to the volcanic ash that is disrupting air travel over Europe," Obama said in a statement released by the White House. "Michelle and I continue to have the Polish people in our thoughts and prayers, and will support them in any way I can as they recover from this terrible tragedy.
"President Kaczynski was a patriot and close friend and ally of the United States, as were those who died alongside him, and the American people will never forget the lives they led," Obama said.
It wasn't clear whether Russian President Dmitry Medvedev, who was in Moscow on Saturday, would fly to Poland for the funeral of the Polish first couple. The Kremlin last week said Medvedev would attend.
Canadian Prime Minister Stephen Harper, South Korean Prime Minister Chung Un-chan and Spanish Prime Minister Jose Luis Rodriguez Zapatero also canceled their plans to attend.
Other leaders, Estonian Prime Minister Andrus Ansip, ditched plane tickets and opted to drive to Poland instead. Latvian President Valdis Zatlers, a close friend of Kaczynski, also planned to make the 13-hour journey from his country to Poland by car.
Eurocontrol, Europe's air traffic authority, said it expects about 5,000 flights to take place Saturday in European air space compared to 22,000 on a normal Saturday. On Friday, there were 10,400 flights instead of the typical 28,000.
The ash cloud was drifting south and eastward over Europe. Although barely visible in the air, the ash -- made up of tiny particles of rock, glass and sand -- poses a serious threat to aircraft.
The volcano was still erupting and spewing ash on Saturday. The eruption began March 20 beneath the Eyjafjallajokull glacier in southern Iceland, blowing a hole in the ice. It worsened over the week, forcing local evacuations and eventually affecting European air space.
There were restrictions on civil flights across most of northern and central Europe, including Austria, Belgium, Croatia, the Czech Republic, Denmark, Estonia, Finland, most of France, most of Germany, Hungary, Ireland, northern Italy, The Netherlands, Norway, Poland, Romania, Serbia, Slovenia, Slovakia, Sweden, Switzerland, Ukraine and the United Kingdom.
The funeral will proceed as scheduled, though it will be affected by the travel restrictions, said Polish Foreign Minister Radoslaw Sikorsky.
"The papal legate who was supposed to lead the funeral mass has apparently canceled," he said. "And quite frankly we can't blame people for not wanting to take even the smallest risk to the security of air traffic in the circumstances."
Several leaders -- including Austrian President Heinz Fischer, Britain's Prince Charles, Swedish King Carl XVI Gustaf and Bulgarian President Georgi Pyrvanov -- plan to attend Kaczynski's funeral if flights resume in their countries.
Britain's Gordon Brown, who has been campaigning to hang on as prime minister since the parliament officially dissolved Monday, was never scheduled to attend. British Foreign Secretary David Miliband was seeking alternate travel plans.
European Commission President Jose Manuel Barroso was in Strasbourg, France, and was seeking alternative travel options to get to Poland.
Meanwhile, thousands of people gathered in the Polish capital on Saturday to mourn the 96 victims of the April 10 plane crash.
Central Warsaw was in lockdown as a trumpeter played, photos of the dead were displayed in Pilsudski Square, and speeches were delivered honoring the victims.
An estimated 1.5 million people are expected to turn out Sunday to bid farewell to Kaczynski, first at a state funeral and then at a church service in the historic city of Krakow.
CNN's Lianne Turner, Caroline Paterson and Antonia Mortensen contributed to this report