WARSAW, Poland — Church bells pealed and emergency sirens blared across Poland at 8:56 a.m. (0656 GMT) Saturday to mark the minute a week earlier that a plane crashed as it attempted to land in Russia, claiming the life of Polish President Lech Kaczynski, his wife and 94 others.Thousands of Poles massed in Warsaw's Pilsudski Square for a public memorial service for the victims. But some world leaders canceled their attendance at a state funeral Sunday, citing the volcanic ash cloud hanging over Europe, leaving numerous airports closed.The crash plunged Poland into a deep grief not seen since the death of Pope John Paul II five years ago.The bells tolling were a precursor to the giant outdoor memorial later Saturday in Pilsudski Square that is expected to draw hundreds of thousands of mourners.The square has been outfitted with large display screens to show the services and big, oversized photos of the 96 people killed are scattered across the site.Acting President Bronislaw Komorowski and Prime Minister Donald Tusk are to speak at the ceremonies in Pilsudski Square, along with Izabela Sariusz-Skapska, daughter of one of the victims.There will be two minutes of silence starting at 12 p.m. (1000 GMT; 6 a.m. EDT) followed by the reading of all 96 victims' names and the playing of the national anthem. A funeral mass will take place afterward.The ceremony is the first of two days of somber ceremonies and will be followed by a funeral mass for the first couple at St. John's Cathedral at 6 p.m. (1600 GMT; noon EDT) in Warsaw.The city operated buses and subways for free and the government banned the sale of alcohol until Saturday night.On Sunday, numerous world leaders are expected for a tradition-laden funeral for Kaczynski and his wife, who were among the 96 people killed when their plane went down in heavy fog after clipping a birch tree on approach to Smolensk, Russia. They had planned to attend a memorial for thousands of Polish army officers executed in 1940 by the forerunner of the Soviet secret police in the nearby Katyn forest.Poland said it expects nearly 100 dignitaries, including U.S. President Barack Obama and Russian President Dmitry Medvedev, for the funeral, but some have had to nix their plans due to the spreading volcanic ash cloud that has choked air travel across Europe, including the representatives of New Zealand, South Korea and India. All airports in Poland remained closed Saturday to flights above the cloud level of 6,000 meters (20,000 feet) because of the ash cloud, including Balice in Krakow where most of the dignitaries are expected to arrive on Sunday morning, said Grzegorz Hlebowicz, spokesman for Poland's aviation authorities.Word of one cancellation for Saturday's ceremony came late Friday. ThePolish news agency PAP reported that Cardinal Angelo Sodano, dean ofthe College of Cardinals, could no longer fly from Rome to deliver amemorial Mass there.PAP quoted unnamed church sources as saying they still hope Sodano willbe able to make it Sunday for the presidential couple's funeral inKrakow, perhaps by flying part of the way and making the last part ofthe trip by car.Sunday's state funeral in mostly Roman Catholic Poland will begin at 2p.m. (1200 GMT; 8 a.m. EDT) with a Mass at the 13th-century St. Mary'sBasilica. The bodies of the first couple will then be carried in afuneral procession across the Old Town to the historic Wawel Cathedral,where they will be interred.