KRAKOW, Poland (Reuters) -- Poland is preparing to bury President Lech Kaczynski and his wife Maria, who were killed in a plane crash last weekend, but a volcanic ash cloud over Europe has prevented many world leaders from attending.
U.S. President Barack Obama cancelled his plan to travel to the funeral in Krakow, Poland's ancient capital, citing the ash cloud. A number of other dignitaries including German Chancellor Angela Merkel and Britain's Prince Charles, also pulled out.
The funeral at Krakow's Wawel cathedral crowns a week of unprecedented national mourning for the Kaczynskis and 94 other, mostly senior, political and military officials who perished in the crash in western Russia on April 10.
Up to 100,000 mourners packed into Warsaw's Pilsudski Square on April 17 for a commemoration of the dead that included a Roman Catholic requiem mass and tributes from Prime Minister Donald Tusk and interim President Bronislaw Komorowski.
Kaczynski's twin brother Jaroslaw, a former prime minister, and other family members insisted the funeral go ahead as planned, even though the volcanic cloud has shut down airports across northern and central Europe, including Poland.
Obama said he regretted not being able to go to Krakow.
"President Kaczynski was a patriot and close friend and ally of the United States, as were those who died alongside him, and the American people will never forget the lives they led," he said in a statement shortly before he had been due to fly.
Poland, part of the Soviet bloc during the Cold War, is now a member of NATO and a close U.S. ally.
Merkel of Germany, Poland's western neighbor and biggest trade partner, also expressed regret at being unable to attend.
Poland's meteorology institute said it expected the ash cloud to partly disperse from over Polish territory on Sunday.
The Kaczynskis, due to be transported to Krakow on aboard a military plane flying at an altitude of less than 5,000 meters because of the ash cloud, will be buried in the Wawel crypt with Poland's kings, national heroes and poets.
Wawel is a large complex of buildings on the Vistula River that includes a castle, cathedral and fortifications.
The cathedral was the coronation site of virtually all of Poland's monarchs. Wawel Castle was the centre of government for five centuries until the end of the 16th century.
Some Poles have staged protest rallies and joined petitions on social media site Facebook against the decision to bury Kaczynski in such a hallowed spot.
Kaczynski, president since 2005, was a polarizing figure whose support levels had fallen to about 20 percent before his death. He had been expected to lose a presidential election due in the autumn and now likely to be held on June 20.
To his conservative admirers, Kaczynski was a patriot and man of deep moral and religious convictions. To his foes, he was a narrow-minded reactionary out of step with an increasingly liberal, outward-looking and European Poland.
The protests were the first cracks in an otherwise remarkable display of national unity since the crash.
Warsaw's quaint Old Town, rebuilt brick by brick after its near-total destruction by the Nazis in 1944, has become a shrine to the 96 dead, bedecked with flowers, candles, crucifixes and red and white national flags threaded with black ribbons.
Kaczynski and his entourage had been heading last Saturday to Katyn forest near Smolensk in western Russia to commemorate the 70th anniversary of the massacre of 22,000 Polish officers and intellectuals by the Soviet secret police.