In this Aug. 12, 2008, file photo, Polish President Lech Kaczynski speaks to the media prior to boarding his plane at the military airport in Warsaw. (Alik Keplicz/Associated Press)Polish President Lech Kaczynski is dead, along with his wife, the head of Poland's central bank and other senior government officials, after the plane they were in crashed Saturday in western Russia in thick fog.

Kaczynski, 60, and his entourage were flying from Warsaw to the Russian city of Smolensk for a memorial service when the aircraft crashed on its final descent, officials said.

The pilot had been asked by ground controllers to divert to one of two alternate airports because of the poor visibility, but persisted and tried to land in Smolensk.

Russian and Polish officials said there were no survivors among the 96 people aboard the presidential aircraft, a 26-year-old Tupolev Tu-154.

'I knew that his whole life had been dedicated to the fight for the freedom of Poland and the freedom of Europe'—German Chancellor Angela Merkel

"We still cannot fully understand the scope of this tragedy and what it means for us in the future. Nothing like this has ever happened in Poland," Foreign Ministry spokesman Piotr Paszkowski said.

Government and military officials were heading to Russia for ceremonies marking the 70th anniversary of the 1940 Katyn Forest massacre, when Soviet secret police killed 22,000 Polish officers.

In addition to the president and his wife, Maria, passengers included Poland's army chief, Franciszek Gagor, Deputy Foreign Minister Andrzej Kremer, central bank governor Slawomir Skrzypek and other officials. Three legislators, the deputy parliament speaker, the army's chaplain, Poland's civil rights commissioner and at least two presidential aides, were also aboard. Nearly all the victims were part of the Polish state delegation for the Katyn commemoration.

The accident was not expected to directly affect the functioning of Polish government. Poland's president is commander in chief of its armed forces but the position's domestic duties are chiefly symbolic.

The prime minister, Donald Tusk, and other top government ministers were not aboard the plane.

Aircraft hit ground, burst into flames

The plane tilted to the left before crashing, witness Slawomir Sliwinski told the state news channel Rossiya-24. He said there were two loud explosions when the aircraft hit the ground and burst into flames.

Rossiya-24 showed footage from the crash site, with pieces of the plane scattered widely amid leafless trees and small fires burning in woods shrouded with fog.

Initial signs pointed to an accident, possibly due to the fog that is very common in the area in spring and fall, as well as pilot error. Both black boxes have been found, and preliminary data indicated that the plane hit the treetops as it was making the approach to the airport in poor visibility, the ITAR-Tass news agency quoted a Russian official as saying.

Andrei Yevseyenkov, spokesman for the Smolensk regional government, said Russian dispatchers had asked the Polish crew to divert from the military airport in North Smolensk and land instead in Minsk, the capital of neighbouring Belarus, or in Moscow to the east because of the fog.

An image from Polish television's TVP shows a firefighter amid the wreckage of the crashed presidential aircraft. (TVP via APTN/Associated Press)

While traffic controllers generally have the final word in whether it is safe for a plane to land, they can and do leave it to the pilot's discretion. Russian Air Force Gen. Alexander Alyoshin confirmed that the pilot disregarded instructions to fly to another airfield. The Smolensk airport is not equipped with an instrument landing system to guide planes to the ground.

According to the Aviation Safety Network, there have been 66 crashes involving Tu-154s, including six in the past five years.

The Russian carrier Aeroflot recently withdrew its Tu-154 fleet from service.

The presidential plane was fully overhauled in December, the general director of the Aviakor aviation maintenance plant in Samara, Russia, told Rossiya-24.

World leaders express shock, sadness

Bronislaw Komorowski, leader of the lower house of parliament, has taken over as acting president and has declared a week of national mourning.

According to Poland's constitution, he must announce a new presidential election within two weeks and the vote — which had been due in October — must take place within two months of Komorowski's announcement.

World leaders sent condolences to Polish officials and praised the president's dedication to democracy.

Poland's ambassador to Canada, Zenon Kosiniak-Kamysz, said the list of government officials is long and "the number of victims is shocking." He was informed of the crash early Saturday and knew many people on the flight.

People mourn in front of the presidential palace in Warsaw on Saturday. (Czarek Sokolowski/Associated Press)

Prime Minister Stephen Harper offered his condolences to Poland and Polish-Canadians, while praising Kaczynski's achievements.

"President Lech Kaczynski was a man who stood proudly and defiantly for democracy and human rights through even the most difficult times. His sudden passing is a great loss for his country and for his many friends here in Canada, particularly within the Polish-Canadian community.

"On behalf of the government of Canada and all Canadians, I wish to express my deepest sympathy, and my sincere condolences, to the government and people of Poland on this very sad day," Harper said.

German Chancellor Angela Merkel said she was shocked and saddened by the death of Kaczynski and other top Polish officials.

“I am so deeply upset by the accident and the death of the Polish president,” Merkel said. "I knew that his whole life had been dedicated to the fight for the freedom of Poland and the freedom of Europe. We will miss Lech Kaczynski in Germany too.

"Germany is mourning today with the whole Polish people."

U.S. President Barack Obama issued a statement saying, "the loss is devastating to Poland, to the United States, and to the world."

British Prime Minister Gordon Brown said Kaczynski was "one of the defining actors in Poland's modern political history.

"We know the difficulties that Poland has gone through, the sacrifices that he himself made as part of the Solidarity movement. We know the contribution he made to the independence and the freedom of Poland," Brown said.

"On this difficult day the people of Russia stand with the Polish people," Russian President Dmitry Medvedev told the Polish prime minister in a telephone call.

Putin to oversee investigation

Polish Prime Minister Donald Tusk flew to the crash site, where he and Russian Prime Minister Vladimir Putin met and laid flowers together.

Putin has personally taken charge of the investigation into the crash, and he offered his condolences Saturday.

"We have to do everything so that in the shortest time we find out the cause of the tragedy," he said.

With files from The Associated Press