Ukrainian President Volodymyr Zelensky and his Polish counterpart commemorated the killing of Poles by Ukrainian nationalists in the Volyn pogrom in 1943.
In a Catholic cathedral in western Ukraine, Mr Zelensky and President Andrzej Duda lay candles in honor of the tens of thousands of those killed.
Poland is one of Ukraine’s closest allies against the Russian occupation.
But the Volhynia massacre remains a dark cloud in their relations.
Ukrainian nationalists, led by Stepan Bandera, operated in German-occupied Poland during World War II and were trained by the Nazis. They attacked about 150 villages and killed the Polish minority.
Poland puts the death toll at 100,000 and says Ukraine should apologize and ask for forgiveness. The Polish parliament had described the massacre as genocide – a description that Ukraine disputes.
The massacre provoked reprisals from Poles against Ukrainian civilians. At least 2,000 Ukrainians were killed.
Sunday’s service was a highly symbolic gesture of reconciliation, as Russian President Vladimir Putin has repeatedly – and without foundation – called Zelensky’s government a Bandera-style “Nazi”.
“Together we salute all the innocent victims of Volyn! Memory unites us! Together we are stronger,” the presidents said in a joint message on Twitter.
As Zelensky said on his official website: “We value every life, remember history, and stand up for freedom together.”
The cathedral service in Lutsk was ecumenical, with the participation of the Orthodox and Catholic clergy.
Poland is one of the main suppliers of military and humanitarian aid to Ukraine.
On Friday, Polish Prime Minister Mateusz Morawiecki visited what used to be the village of Ostrowki. He honored the victims of Volhynia by erecting a wooden cross in a field and laying a wreath at the monument.
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