Poland's President Lech Kaczynski, its central bank head and the country's military chief were among 96 people killed when their plane crashed in thick fog on its approach to a Russian airport on Saturday.
The president's wife and several other high-ranking government officials were also aboard the Tupolev Tu-154 that plunged into a forest about two km (1.3 miles) from the airport in the western Russian city of Smolensk.
"The political consequences will be long-term and possibly will change the entire future landscape of Polish politics," said Jacek Wasilewski, professor at the Higher School of Social Psychology in Warsaw.
Polish government spokesman Pawel Gras said the country would hold elections after the death of Kaczynski, who was 60 and had been president since 2005.
"In line with the constitution, we will have to hold an early presidential poll," Gras said. "For now, the speaker of the lower house of parliament, Bronislaw Komorowski, is automatically "¦ the acting president."
Russian television showed the smoldering fuselage and fragments of the plane scattered in a forest. A Reuters reporter saw a broken wing some distance from the rest of the aircraft.
Russia's Emergencies Ministry said 96 people were aboard the government plane, including 88 members of a Polish delegation en route to commemorate Poles killed in mass murders in the town of Katyn in 1940.
Earlier reports had said 132 people were aboard. Smolensk regional governor Sergei Antufyev and Polish state news agency PAP said there were no survivors.
Pilot error was a possible reason for the crash, said Andrei Yevseyenkov, spokesman for the Smolensk local government. Local officials said the plane had clipped treetops on its way down.
A Russian mission control official who had been present during conversations with the pilot told Reuters the pilot had ignored advice.
"The pilot was advised to fly to Moscow or Minsk because of heavy fog, but he still decided to land. No one should have been landing in that fog," he said, on condition his name was not published.
Flowers and candles
Polish Justice Minister Krzysztof Kwiatkowski said he would order a special inquiry into the crash.
Church services were hastily held throughout Poland. In Warsaw people started gathering outside the presidential palace to lay flowers and light candles.
"I'm all broken up ... it cannot be expressed in words," said Ewa Robaczewska.
Among the other casualties of the crash were Kaczynski's wife Maria, along with Slawomir Skrzypek, 47, who had been central bank governor since 2007, the chief of Poland's military Franciszek Gagor and Deputy Foreign Minister Andrzej Kremer.
Some relatives of victims of the Katyn massacres were also on board, said a Polish government official in Smolensk.