Hundreds of thousands of mourners are attending a memorial for victims of Poland's air crash tragedy.

The service in Warsaw comes a week after a plane carrying president Lech Kaczynski and 95 others crashed in Russia, killing all on board.

Mr Kaczynski and his wife are set to be buried tomorrow in the southern city of Krakow.

However, the volcanic ash cloud from Iceland that has disrupted air travel could prevent world leaders and dignitaries from attending.

Church bells rang across the country at exactly 8.56am (7.56am Irish time), the time a week ago when the plane came down.

The four-hour memorial service is being held in Pilsudski Square, which is also home to Poland's tomb of the unknown soldier.

It has been the traditional site of national events including a mass held by late Polish pope John Paul II when he visited his homeland in 1979.

During the past week mourners have covered the square with coloured candles, while a huge stage with black and white photos of the dead has been set up.

The presidential jet came down while carrying a delegation to a ceremony for the 70th anniversary of the Katyn massacre, when 22,000 Polish officers were slaughtered by Soviet forces in World War II.

Victims of the crash included the country's military chief, the heads of all three armed forces, the governor of the central bank and the head of the country's Olympic committee.

After today’s mass, the coffins of Mr Kaczynski and his wife Maria, who have been lying in state in the presidential palace since Tuesday, will be taken to nearby St John's Cathedral.

Warsaw's Archbishop Kazimierz Nycz will lead a service followed by an overnight vigil.

The bodies will arrive in Krakow tomorrow morning for the funeral in the cathedral of Krakow's hilltop Wawel Castle, where Poland's past kings and national heroes already lie.