(WATCH: The wreckage of the plane crash. Source: Reuters.)
(RFE/RL) -- Poles are in mourning today following the death of President Lech Kaczynski and scores of other Polish government, military, and church officials in a plane crash in western Russia.
A week of mourning has been called in Poland and a two-minute silence will be held at 1200 local time. Kaczynski's body is expected to arrive back in the country today.
Bronislaw Komorowski, the head of Poland's lower house of parliament, has taken over as interim head of state.
Investigations have been launched in Russia into why the presidential jet, an older Soviet Tupolev Tu-154, clipped the tops of trees, crashed and broke up in flames on April 10 as it approached the runway at the Smolensk airport.
Russian officials have suggested that heavy fog or an error by the Polish crew may have played a role in the crash, which killed all 97 passengers and crew on board. Both "black box" flight data recorders from the aircraft have been found.
In a cruel twist of fate, the delegation -- which included much of the upper echelons of the country's political and military elite -- was en route to pay homage to the victims of the Katyn massacre, when thousands of Polish military officers and civic leaders were executed by the Soviet secret police in April 1940 during World War II.
President Kaczynski had wanted to attend the April 7 commemorations at Katyn, headed by Polish Prime Minister Donald Tusk and Russian Prime Minister Vladimir Putin. But Kaczynski -- a frequent critic of the Kremlin -- did not receive an invitation from Moscow, prompting his trip on April 10.
Among the other officials on board the Tupolev Tu-154 aircraft and presumed dead were Central Bank governor Slawomir Skrzypek, Army Chief of Staff General Franciszek Gagor, and Deputy Foreign Minister Andrzej Kremer.
Also on board were the head of the National Security Office, the deputy parliamentary speaker, the civil rights commissioner, two presidential aides, and three lawmakers. (Click here for a list of the known dead so far.)'This Tragic, Cursed Katyn'
Aleksander Kwasniewski, Kaczynski's predecessor as president, bitterly noted what he called "the horrible symbolism" of the two tragedies, which took place seven decades apart. "This tragic, cursed Katyn," he said. "It's hard to believe. You get chills down your spine."
In an emotional address to the Polish people, Russian President Dmitry Medvedev expressed his condolences, declared April 12 a day of national mourning in Russia, and promised to work closely with the Polish authorities in investigating the tragedy.
"In recent days, we together conducted memorial services in Katyn. We together mourned the victims of totalitarian times. Lech Kaczynski was flying to Russia to personally pay homage to the fallen Polish officers, as a president and as a citizen of his country. All Russians share your grief and mourning," Medvedev said.
Medvedev placed Prime Minister Vladimir Putin, who visited the scene, in charge of the investigation of the crash.
Journalist Marcin Wojciechowski, a correspondent for the Polish daily "Gazeta Wyborcza" spoke to RFE/RL from the scene of the crash: "Rescuers are continuing their work. They aren't moving the remains of the dead yet so experts can investigate the causes of the tragedy. Police and prosecutors are on the scene. Firefighters put out the blaze fairly quickly."
In Warsaw, Prime Minister Donald Tusk called an extraordinary meeting of his cabinet. Tusk called the crash, "the most tragic event in Poland's postwar history" and said he would immediately fly to the crash site in Smolensk. He also reassured Poles that despite the loss of so many top officials, the state will continue to function.
Kaczynski's death marked the first time a Polish president has died in office since 1922, when Gabriel Narutowicz was assassinated by a nationalist after being in office for just five days.Mourning
In Poland, tens of thousands of mourners have turned out in central Warsaw to lay candles and flowers outside the presidential palace. Black ribbons appeared in many windows in the Polish capital.
Adrianna Parowska was among those who gathered to pay their respects. "I came here and like others from Warsaw, we are all in shock. We are here to be together, to console each other. It's a great shock because the elite of our nation went there to remember the dead from 70 years ago," she said.
"Right now Poland is really in trouble. We are all in shock and our thoughts are with the families of the dead. Actually, it's very hard to say anything."
In Russia, the Rossiya-24 television channel showed hundreds of people around the monument to the victims of the Katyn massacre, many holding Polish flags, some weeping.
Kaczynski, 60, is survived by his daughter Marta and two granddaughters. He became president in December 2005 after defeating Tusk in that year's presidential election.
Kaczynski and his twin brother Jaroslaw have been dominant forces in Polish politics for years. Jaroslaw Kaczynski was not on board the plane that crashed. Polish media said he was due to visit the crash site.
Kaczynski 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.
His death brings political uncertainty. A presidential election had been due in October but now must be held within two months, according to the constitution. In a televised address, Poland's acting President Komorowski said he will announce the date of the election after talks with all political parties.
Kaczynski had said that he planned to seek a second term in elections scheduled for October. He was expected to face a difficult race against Komorowski, the candidate of Tusk's governing Civic Platform party.
While the president's role is largely symbolic, the holder can veto government legislation. Kaczynski infuriated the government of Prime Minister Donald Tusk several times by blocking key legislation.Spotty Safety Record
The presidential aircraft, a Tu-154 painted in the official Polish colors of red and white, was at least 20 years old.
Investigators, who have recovered the plane's two flight data recorders, are trying to determine the cause of the crash, which was recorded at 10:50 am local time.
According to the Aviation Safety Network, there have been 66 crashes involving Tu-154s, including six in the past five years. Russian carrier Aeroflot recently withdrew its Tu-154 fleet from service. Polish officials have long sought to replace the ageing planes that carry the country's leaders but said they lacked the funds.
Possible pilot error is being examined. Lieutenant General Alexander Alyoshin, the deputy head of Russia's air force, told Russian news agencies that air traffic controllers had repeatedly advised the pilots to land at another airport, due to the poor visibility. But he said the crew ignored the warnings.
World leaders have expressed shock and sadness over the deaths of President Kaczynski and the Polish delegation, with many statements paying tribute to the Polish leader's patriotism and support for freedom.
Speaking to journalists in Lochgelly, Scotland, British Prime Minister Gordon Brown said "the whole world will be saddened and in sorrow" over the deaths of Kaczynski, his wife, and the other passengers on the flight.
The White House said U.S. President Barack Obama has called Tusk -- whom he had just met in Prague on April 8 at a dinner with Eastern European leaders -- to express the "deepest condolences to the people of Poland."
"Today’s loss is devastating to Poland, to the United States, and to the world. President Kaczynski was a distinguished statesman who played a key role in the Solidarity movement, and he was widely admired in the United States as a leader dedicated to advancing freedom and human dignity," Obama said in a written statement released by the White House.