Poland's President Lech Kaczynski, its central bank head and the country's military chief were among 97 people killed when their plane crashed in thick fog on its approach to a Russian airport.
Polish Prime Minister Donald Tusk described the crash as ‘the most tragic event of the country's post-war history’.
Ashen-faced and wearing a black suit and tie, Mr Tusk told a news conference he would fly to the crash site.
Tributes and messages of condolences have flooding in from across the world, while the Polish community in Ireland has held vigils to mourn the loss of their president.
Russian Prime Minister Vladimir Putin talked to Mr Tusk by telephone and has also gone to the scene of the crash, a spokesman said.
The death of Mr Kaczynski, who with his twin brother was a dominant force in Polish politics, brings political uncertainty.
A presidential election had been due in October but now must be held within two months, according to the constitution.
The president's wife and several other high-ranking government officials also were aboard the aged Tupolev Tu-154, which plunged into a forest about 1.3 miles from the airport in the western Russian city of Smolensk.
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.
Kaczynski, 60, was a one-time ally of Solidarity hero Lech Walesa and a co-founder of the rightist Law and Justice party with his brother. He resigned from the party when he became president in 2005 but continued to support it.
A party official told Reuters the president's twin, Jaroslawi, had left for Smolensk.
Kaczynski's death, along with that of many senior members of Law and Justice who also were on board, at a stroke changes the Polish political scene by wiping out much of the opposition.
While the president's role is largely symbolic, the holder can veto government legislation. Lech Kaczynski infuriated Mr Tusk's government several times by blocking legislation including health sector reform.
The speaker of the lower house of parliament, Bronislaw Komorowski, has been named acting president, as the constitution stipulates. Mr Komorowski is also Tusk's presidential candidate in the centrist Civic Platform party.
Russian television showed the smouldering fuselage and fragments of the plane scattered in a forest.
The plane was one of two Tu-154s in the government fleet, both about 20 years old. Government officials had complained about the age of Poland's official fleet.
Russia's Emergencies Ministry said 97 people were aboard, including 88 members of a Polish delegation en route to commemorate Poles killed in mass murders in the town of Katyn under orders from Soviet leader Josef Stalin in 1940.
Smolensk regional governor Sergei Antufyev said there were no survivors of the crash. The Emergencies Ministry said the bodies of the victims would be transported to Moscow for investigation.
Polish Justice Minister Krzysztof Kwiatkowski planned an inquiry into the crash. Russian President Dmitry Medvedev said Russian investigators would cooperate with the Polish side.
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 also were on board, said a Polish government official in Smolensk.
Thousands of Polish prisoners of war and intellectuals were murdered at Katyn by Soviet forces in spring 1940 in an enduring symbol for Poles of their suffering under Soviet rule.
The Polish government has declared a week of national mourning.
Jan Kaminski, a spokesperson for the Irish Polish Society, has said
questions must be asked why so many 'important people' chose to travel on such an 'antiquated aircraft', heading in the direction of a 'provincial airport'.
Mr Kaminski said he was 'paralyzed with surprise and numbed' after hearing the news.