KIEV, April 11 – President Viktor Yanukovych declared a day of mourning Monday following the death of Polish President Lech Kaczynski, and dozens of Polish political and military leaders, in a plane crash in western Russia.

Yanukovych announced the decision on Sunday while visiting the Polish Embassy to offer his condolences. Kaczynski was widely considered to be Ukraine’s key advocate in the European Union.

Former Prime Minister Yulia Tymoshenko, the leader of the largest Ukrainian opposition group, said the crash was a “terrible tragedy.”

“A true friend of Ukraine died in the plane crash,” Tymoshenko said.

Russia and Georgia also declared a day of mourning on Monday.

“Poland is our long-time friend,” Taras Chornovil, a lawmakers from Yanukovych’s Regions Party, said. “The president of Poland was a frequent guest in Ukraine.”

Kaczynski, 61, was arriving in Smolensk for a ceremony to commemorate the 70th anniversary of the murder of more than 20,000 Polish officers by the Soviet secret police after the Red Army invaded Poland. Soviet-designed aircraft, Tupolev Tu-154, was landing in heavy fog.

A spokesman for Poland's ministry of foreign affairs said 88 people were on the plane. Russian emergency officials said the total number killed, including crew members, was 96.

Polish Parliament Speaker Bronislaw Komorowski, a member of the Civic Platform party of Prime Minister Donald Tusk, who would likely have challenged Kaczysnki in upcoming elections, must by constitutional law announce early elections within 14 days of the president's death. The vote must be held within another 60 days.

"This is the most tragic event in the history of Poland outside wartime," Tusk said in a televised speech in Poland. "Such a dramatic event is unprecedented in the modern world."

Kaczysnki was elected president in 2005 just as his identical twin brother, Jaroslaw, became head of the nationalist-conservative Law and Justice government, often putting Poland on a collision course with Russia.

Kaczynski forged close relationships with Ukraine and Georgia and pushed for their accession into NATO, arguing passionately that a stronger NATO would keep Russia from reasserting its influence over Eastern Europe.

Among those on board, according to the Web site of the newspaper Gazeta Wyborcza, were Kaczynski; his wife, Maria; former Polish president-in-exile Ryszard Kaczorowski; the deputy speaker of Poland's parliament, Jerzy Szmajdzinski; the head of the president's chancellery, Wladyslaw Stasiak; the head of the National Security Bureau, Aleksander Szczyglo; the deputy minister of foreign affairs, Andrzej Kremer; the chief of the general staff of the Polish army, Franciszek Gagor; the president of Poland's national bank, Slawomir Skrzypek; the commissioner for civil rights protection, Janusz Kochanowski; the heads of all of Poland's armed forces; and dozens of members of parliament. (tl/ez)