President Andrzej Duda and Prime Minister Mateusz Morawiecki attended an event at a Warsaw monument honouring the victims of wartime killings known as the Volhynia Massacres.
Duda said during the ceremony that the Poles and Ukrainians "today need great responsibility for the future" to put their difficult past behind them.
He added that, even though the hostilities decades ago were a "shameful" chapter in bilateral relations, he believed Poland and Ukraine would "cope with this difficult challenge" and put in place "a new kind of truly neighbourly and fraternal" cross-border ties.
He noted that thousands of Poles were hosting their Ukrainian neighbours in their homes and helping Ukraine amid Russia's invasion.
A delegation of Ukrainian officials led by the country's incoming ambassador to Warsaw, Vasyl Zvarych, took part in the commemoration on behalf of President Volodymyr Zelensky to honour those who lost their lives 79 years ago, public broadcaster Polish Radio’s IAR news agency reported.
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.
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Source: IAR, PAP