Poland has unveiled a new monument on the 70th anniversary of the Volyn massacre, in which nationalist Ukrainian partisans killed about 40,000 Polish civilians during World War II.
At the time Volyn - also called Volhynia - was an area of south-eastern Poland occupied by Nazi German troops.
The 1943 atrocity was aimed at driving out the Polish minority. On 11 July alone 10,000 people were murdered.
Poles later retaliated and the conflict killed up to 100,000 people in total.
The violence also displaced some 1.5 million people from their homes.
Polish President Bronislaw Komorowski unveiled a monument to victims of the massacre at an outdoor Mass in Warsaw, the BBC's Adam Easton reports from the Polish capital.
The nationalist Ukrainian Insurgent Army (UPA) carried out the massacre. The army was anti-Soviet and pushing for Ukrainian independence.
After the war Volyn was incorporated into the Soviet Union while Poland's western border was shifted further west.