Official ceremonies being held a year after a plane crash that killed Poland's President Lech Kaczynski have been boycotted by his twin brother.
Jaroslaw Kaczynski has condemned the government's handling of the affair, and held his own rival events.
After initially bringing the country together, the crash is now bitterly divisive, the BBC's correspondent says.
Official events began at 0841 local time, the moment when the plane crashed in Smolensk, Russia, on 10 April 2010.
Poland's new president, President Bronislaw Komorowski, silently paid tribute to his predecessor and the 95 other victims at a memorial in a Warsaw church.
He and Prime Minister Donald Tusk placed candles at the foot of the memorial.
Call for healing
Later, at the cemetery where many of the crash victims have been buried, Mr Komorowski recalled the moment when "for many of us... the world collapsed".
He called for healing, rather than recrimination, in the wake of the national tragedy.
"One of the finest memorials we can build together is to care for the dreams and passion of those who died, so that they find followers who will carry their passion, dreams and hopes into the future," the Associated Press reported him saying.
Meanwhile, Jaroslaw Kaczynski was leading unofficial commemorations.
The former prime minister, who founded the conservative Law and Justice Party along with his identical twin, laid a wreath outside Warsaw's presidential palace.
Thousands of supporters held Polish flags, near a makeshift shrine decorated with a cross and a model of Poland's presidential Tupolev 154 plane broken in two.
Jaroslaw Kaczynski, like many Poles, has refused to accept the findings of the Russian investigation into the crash, which effectively blamed the plane crew for deciding to land despite dense fog and warnings from ground controllers.
He says the Russian authorities should have closed the airport.
The affair has tested the improving diplomatic relationship between Poland and Russia. The latest row has been caused by Russia's decision to replace a sign at the crash site, which referred to the Katyn "genocide".
The Polish delegates had been on their way to a memorial for the victims of Katyn, where 20,000 Polish officers were massacred by Soviet forces in 1940.