Presidents Andrzej Duda and Volodymyr Zelensky took part in the commemorations at Lutsk's Cathedral of the Apostles Peter and Paul.
The Polish President’s office posted a photo of the two presidents walking in the cathedral with lit candles in a social media post, saying: "Together we pay tribute to all the innocent victims of Volhynia! Remembrance unites us! Together we are stronger!"
Volodymyr Zelenski also published photos from the ecumenical service on his social media channels, with a similar caption.
The Volhynia Massacres were carried out between February 1943 and the spring of 1945 by the Ukrainian Insurgent Army (UPA) in Nazi German-occupied Poland, according to Poland’s National Institute of Remembrance (IPN).
Some 100,000 ethnic Poles in total were slaughtered in the 1940s by Ukrainian forces, according to some estimates.
On July 11, 1943, the day of the worst bloodshed, Ukrainian nationalists attacked 100 villages largely inhabited by Poles in what was then Nazi-occupied eastern Poland and is now western Ukraine.
The massacres were part of an operation carried out by the Ukrainian Insurgent Army (UPA), whose plan was to have a sovereign and nationally homogenous Ukraine after the war.
The Volhynia region, which was within Poland's borders prior to World War II, was first occupied by the Soviets in 1939, and then by the Nazi Germans in 1941.