polandWARSAW (AFP) - – More than 100,000 people gathered Saturday to honour Poland's president and 95 other victims of an air crash, as world leaders cancelled plans to attend his funeral because of a volcanic ash cloud.

US President Barack Obama, German Chancellor Angela Merkel and other foreign dignitaries said they could not come to Sunday's burial service because of the plume from an Icelandic eruption that is causing travel chaos across Europe.

The news came as mourners flocked into Warsaw's main Pilsudski Square for a huge memorial service for Polish president Lech Kaczynski and scores of top officials killed when their plane crashed in Russia one week ago.

A huge altar in the square with a giant white cross displayed black and white photographs of all 96 who perished, as an actor solemnly read the name of each victim, starting with Kaczynski and his wife Maria.

"Things like this never happen, they are impossible. It is the greatest tragedy in the history of Poland since World War II," Prime Minister Donald Tusk told mourners after a lone bugler sounded a funeral air.

Mourners waved red and white Polish flags decked with black ribbons. They applauded Kaczynski's identical twin brother, former premier Jaroslaw, and the couple's daughter Marta as they arrived.

Sirens wailed at 8:56 am (0656 GMT) -- the exact time when the presidential jet crashed en route to a ceremony marking the 70th anniversary of the Soviet massacre of thousands of Polish officers -- and again when the service started.

The dead also included the country's military chief, the heads of all three armed forces, the governor of the central bank and the boss of its Olympic committee, as well as iconic opponents of Poland's communist-era regime.

The square is the traditional site for national services such as the mass of late pope John Paul II when he visited his deeply Catholic homeland in 1979 and also when Pope Benedict XVI came to Poland in 2006.

The memorial was followed by a funeral mass for the presidential couple at nearby St. John's Cathedral in Warsaw's old town.

Onlookers clapped as their coffins, which have been lying in state in the presidential palace since Tuesday, moved slowly past on gun carriages, and a military band played the funeral march by Franco-Polish composer Frederic Chopin.

"We needed to be here in this tragic time," said Jan Szylborski, who came from a small town on buses organised by Solidarity, the trade union that helped bring down communism in Poland in 1989 and in which Kaczynski was an activist.

Elected in 2005, the conservative nationalist Kaczynski was a divisive figure at home and abroad, but the crash has brought unity to Poland's fractious political scene, as well as rapprochement with historic foe Russia.

Acting president Bronislaw Komorowski said at the public memorial that Poland was "grateful to the citizens of Russia who have spontaneously conveyed their compassion to Poland and the Polish people."

A military aircraft will take the bodies of the Kaczynskis to the southern city of Krakow early Sunday for the funeral and burial in the cathedral of the hilltop Wawel castle, where Poland's past kings and national heroes are buried.

But the volcanic ash cloud hit the funeral preparations, as Obama, Merkel Canadian Prime Minister Stephen Harper, and Spanish premier Jose Luis Rodriguez Zapatero all said they could not come.

Britain's Prince Charles and Foreign Minister David Miliband also cancelled.

It was not immediately clear Saturday evening whether Russia's president Dmitry Medvedev would also be forced to skip the ceremony due to the ash cloud, which has forced a no-fly zone over much of the continent.

Other European leaders Saturday began a long drive to Krakow, including those from Poland's fellow eastern members of the European Union who recognised Kaczynski as a defender of the region's interests against Western giants.

"Kaczynski was a fervent proponent of equality. He stood up for everybody, not just himself," said Estonian Prime Minister Andrus Ansip, who was to drive for 18 hours.

Russian and Polish investigators are continuing to probe the cause of the crash, with Russian officials saying they suspect pilot error as the Tupolev Tu-154 plane tried to land in fog near Smolensk in western Russia.