President Lech Kaczynski and his wife were part of an 88-person state delegation headed to events marking the 70th anniversary of the Red Army's murder of thousands of Polish military officers near the Russian village of Katyn during World War II.
Russian officials said all 97 people aboard perished after the jet clipped trees as it came in for landing near Smolensk under heavy fog.
"This is unbelievable -- this tragic, cursed Katyn," former Polish president Aleksander Kwasniewski told Polish television. "It's hard to believe. You get chills down your spine."
Television footage showed smoldering segments of the Tupolev jet's fuselage scattered among the trees. A Russian government spokesman said dispatchers had told the pilot to divert to another airport because of poor visibility, but the crew opted to land anyway.
In addition to the president, those killed included top commanders in all four branches of the Polish military, the president of Poland's national bank, the head of the National Security Office, and a number of lawmakers and top presidential aides. Also onboard, relatives of those killed in the forests of Katyn in 1940.
"Our thoughts and prayers are with the Kaczynski family, the loved ones of those killed in this tragic plane crash, and the Polish nation," President Obama said in a White House statement.
Mourners flocked to the Presidential Palace in Warsaw, laying flowers and lighting candles. Prime Minister Donald Tusk called for two minutes of silence today -- and the speaker of the Polish parliament, who was scheduled to challenge President Kaczynski this fall, assumed the role of acting president. The Polish constitution calls for emergency elections in 60 days.
Kaczynski, who was 61, won the presidency in 2005, just as his identical twin brother Jaroslaw took charge of the nationalist-conservative Law and Justice Party and became the country's prime minister.
Several news organizations are tracking the fallout of the crash:
The New York Times has this round-up of reactions from world leaders.
NPR's Two-Way blog has been tracking developments.
And the English language service of Polskie Radio covers the story here.
And finally there's video of the crash scene on YouTube.