Warsaw, Poland (CNN) - World leaders' plans to reach Poland this weekend for the funeral of President Lech Kaczynski were in limbo as volcanic ash from Iceland continued to ground flights to and from Europe.
While French President Nicolas Sarkozy was already in Poland on Saturday and prepared to attend services the next day, other key leaders who planned to attend, including German Chancellor Angela Merkel and U.S. President Barack Obama, still faced possible cancellations due to the ash. The White House said Obama was scheduled to depart from Andrews Air Force Base late Saturday, but officials were still monitoring the situation.
Polish President Lech Kaczynski, his wife, dignitaries and top military leaders were killed when their plane crashed in bad weather. They were on their way to a service commemorating Polish prisoners of war massacred in Russia during World War II.
It wasn't clear whether Russian President Dmitry Medvedev, who was still in Moscow on Saturday, would take a flight to Poland for the funeral of the Polish first couple. The Kremlin last week said Medvedev did plan 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 normal 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 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.
The funeral Sunday 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 resumed 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. Canadian Prime Minister Stephen Harper still planned to attend.
European Commission President Jose Manuel Barroso was in Strasbourg, France, and was seeking alternative travel options to get to Poland.
South Korean Prime Minister Chung Un-chan and Spanish Prime Minister Jose Luis Rodriguez Zapatero canceled their plans to attend.
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 a 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