Poland is marking the fifth anniversary of a plane crash in Russia that killed the Polish president and several other senior officials.
The crash, in Smolensk, western Russia, killed 96 people, including Polish President Lech Kaczynski, his wife, the central-bank governor, top army commanders, and other high-ranking officials.
The high-level delegation was traveling to Russia to commemorate victims of the World War II massacre of Polish military officers at Katyn by members of the Soviet secret police.
The late president's daughter, Marta Kaczynska, placed flowers early on April 10 on her parents' grave at Wawel Cathedral in the city of Krakow.
In Warsaw, President Bronislaw Komorowski, Prime Minister Ewa Kopacz, and other leaders attended a memorial ceremony at the Powazki military cemetery, gathering next to a memorial shaped like airplane wings entering the ground.
Earlier, parliament speaker Radek Sikorski and other officials placed wreaths at the parliament to remember the 18 lawmakers who died in the crash.
Many other commemorations are scheduled throughout the day, including a ceremony in the capital to be attended by the president, prime minister, and other leaders.
Five years ago, the high-level delegation was traveling to Russia to remember the victims of a World War II massacre of Polish military officers at Katyn by members of the Soviet secret police.
A Polish investigation into the causes of the crash is still under way.
Last month, Polish prosecutors said they planned to bring charges against two Russian air-traffic controllers over the crash. Prosecutors said the controllers were involved in communicating with the plane as it tried to land in bad weather.
For its part, Russia has denied that air-traffic controllers were to blame. A statement this week by a Russian Investigative Committee said it "sees no grounds to talk of even minimal responsibility of the flight-control group for the air crash."
The committee said it blamed a combination of factors, including the decision by the Polish crew not to land at a different airport in poor weather and the crew's failure to react to automated system warnings during descent.
A leaked Polish transcript has suggested the commander of the Polish Air Force, one of the traveling dignitaries, was in the cockpit during the flight and may have pressured the pilots into landing in thick fog.