The president's wife Maria and several other high-ranking government officials, who were seen as the nation's elite, were also on board the Soviet-built plane, a Tupolev Tu-154.
The plane hit a tree before landing, losing the rear part of the fuselage a few hundred metres before the runway, local officials said.
Russian television showed smoldering fragments of the plane scattered across a forest. Fog was reported in the area when the plane crashed at 10:56 Moscow time (8:56 CET).
Among the casualties were Slawomir Skrzypek, Poland's central bank governor since 2007, Franciszek Gagor, the chief of the country's military, several military commanders and Deputy Foreign Minister Andrzej Kremer. Relatives of the victims of the Katyn massacre were also on board.
"A second Katyn has stricken Poland. The elite of our country has perished," said Lech Wałęsa, former president of Poland and iconic leader of the anti-communist movement Solidarność.
"It is the biggest tragedy, [and] not only in Polish history," said historian Antoni Dudek.
"Katyn remains a tragic place for Poles," said Ryszard Kalisz from the Alliance of the Democratic Left (SLD).
EurActiv Poland reports that the last Polish president in exile, Ryszard Kaczorowski, was also on board, together with Anna Walentynowicz, a brave woman who started the 1980 strike in Gdańsk together with Lech Wałęsa. German director Volker Schlöndorff made a film based on her biography a few years ago ('Strajk: The Hero from Gdańsk/Danzig').
Poland 'in shock'
Polish Prime Minister Donald Tusk cried when he heard the news, said Foreign Minister Radosław Sikorski.
Lech Kaczyński, 60, became president in December 2005 after defeating Tusk in that year's presidential election. Kaczyński was planning to run for a second term in fresh polls due this autumn.
Parliament speaker Bronisław Komorowski will take over presidential duties, according to the Polish constitution. Komorowski, who is from Tusk's Civic Platform party, was expected to be Kaczyński's main opponent in the presidential race.
Seven days of national mourning were announced. The inhabitants of Warsaw are expected to gather at 20:30 tonight on Pilsudski Square in central Warsaw. A similar reflective mood gripped the Polish nation after the death of Pope John Paul II five years ago.
In competition with Tusk?
Three days ago, Polish Prime Minister Donald Tusk was in Katyn alongside his Russian counterpart Vladimir Putin to commemorate the massacre. The Katyn commemoration was seen as a sign of steadily improving relations between Russia and Poland (EurActiv 08/04/10).
Putin had invited Tusk himself, while Kaczyński, who was seen less sympathetically in Russia, decided to go uninvited three days later. According to press reports, details of his trip were hanging in the air until the last moment, as the Russian Foreign Ministry claimed it had not been properly notified about Kaczyński's intention to visit the Katyn massacre site.
To make up for the lack of Russian representatives, Kaczyński had decided to surround himself with a large delegation, Polish sources told EurActiv.
Jedrzej Bielecki, a Polish journalist working for the Dziennik daily, told EurActiv from Warsaw that his country was in shock. The Polish presidential palace located in the historic centre became an improvised mourning site, with thousands of Polish citizens laying flowers, lighting candles and praying, he said.
"For the time being, people are living this difficult moment in a religious way. There will be many church services this afternoon. But people would also like to know what the cause of the accident was, and some elements are emerging," Bielecki said.
Pilot under pressure?
Bielecki said Polish citizens were eager to know why the plane commander had attempted to land at Smolensk. Before it crashed, the plane had made three unsuccessful attempts to land at Smolensk airport, which was not fitted with a modern navigation system, he said.
The plane commander had been advised to land 400km away in Minsk, Belarus, Bielecki said. "We expect the black boxes to reveal if there was pressure on the commander to land in Smolensk," he added.
Bielecki has travelled several times with the presidential airplane. He said that on a previous occasion, when Kaczyński was travelling to Georgia in 2008, the commander refused to land in Tbilisi because the airport was not fitted with modern navigation systems.
"The pilot landed in Baku [in neighbouring Azerbaijan] and Kaczyński threatened to have him dismissed from his job," he said. Four years ago, Bielecki said one of the engines of the same plane caught fire when he was accompanying Finance Minister Marek Belka to Vietnam.
"The big question is: did the president urge the pilot to land in Smolensk. Was there such pressure?"
Bielecki said it was a paradox that Poland was the first former communist country to get rid of its Soviet fleet. It scrapped it in 1984 following the crash of an IL-62 in Warsaw from Polish company LOT.
"LOT was the first airline of a communist country to change its Soviet planes for Boeings, because Polish citizens did not want to use Soviet planes. The paradox is that the leadership kept the last two Tupolev planes as the official fleet. And it did not dare to change them, because it would have been accused of over-spending in times of crisis," he said.
"When [Foreign Minister Radosław] Sikorski went to Kiev with his German colleague recently, they took a German plane because the Germans categorically refused to use the Tupolev plane," Bielecki further explained.
Another detail that Bielecki pointed out was that Kaczyński's plane was not on course to land when it crashed.
"The plane was found 400 metres from the runway, but seventy metres to one side. It was off-course. Even it had not hit a tree, it would have crashed," the Polish journalist said.
The Russian press wrote that the Polish presidential plane, which was built in 1990, was recently modernised by Russian company Aviakor in the city of Samara.