More than 100,000 Poles gathered in Warsaw's historic main square Saturday for an emotional memorial service to honour president Lech Kaczynski and 95 others who died in an air crash in Russia.
The ceremony came as a volcanic ash cloud from Iceland caused several world leaders to cancel plans to fly to Kaczynski's state funeral in Krakow on Sunday, although US President Barack Obama was still set to come.
Sirens wailed at 8:56 am (0656 GMT) -- the exact time one week ago when the presidential jet crashed in western Russia -- and again when the service started at noon, bringing cars and pedestrians to a halt.
"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.
A huge altar in Pilsudski 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.
The air disaster scythed through the nation's political and military elite as they headed to a ceremony marking the 70th anniversary of the wartime massacre by Soviet forces of 22,000 Polish officers.
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.
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.
"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.
Warsaw police estimated the crowd at more than 100,000 people.
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.
"Warsaw and Poland have passed the test of solidarity and patriotism," said Kazimierz Nycz, the Archbishop of Warsaw who led the mass.
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.
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 cloud of ash from an Icelandic volcano which has disrupted European air travel hit preparations for Sunday's funeral in the southern city of Krakow, with Canadian Prime Minister Stephen Harper, Spainish Prime Minister Jose Luis Rodriguez Zapatero and Britain's Prince Charles cancelling plans to attend.
Other European leaders began Saturday a long drive to Krakow.
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.
Up to one million people are expected to turn out in Krakow.
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.