Eighty years ago, Nazi troops discovered in Russia's Katyn forest the bodies of thousands of Polish army officers killed by Soviet secret police in 1940 -- a massacre the Kremlin denied until 1990.
"It is one of the most tragic episodes of our history, casting such a terrible shadow over Polish-Russian relations," President Andrzej Duda said in Warsaw, after laying a wreath in memory of the victims.
"Today we look on in horror at... all the records and video and photos coming out of Bucha and other places in Russian-occupied Ukraine where people were murdered the same way, in the Katyn manner," he added.
Prosecutors in Kyiv say Russian forces killed some 1,400 civilians around Bucha, a town near the Ukrainian capital, where the bodies were discovered last year after the withdrawal of Russian troops.
Bucha has since become a symbol of the alleged war crimes carried out by Moscow during its invasion of Ukraine.
"The perpetrators of the crimes carried out by Russians in Ukraine over the past year must be held accountable so that crimes like the Katyn massacre, like the crimes carried out by Russians in Ukraine today, may never happen again," Duda said.
Poland, a member of NATO and the European Union, has been a staunch Ukraine supporter -- through words and weapons -- since Russia's invasion last year.
The Katyn massacre is also associated with another Polish tragedy, the 2010 air crash in Smolensk, Russia.
Poland's president and many senior state officials had been heading to a ceremony to mark the Katyn massacre when their plane crashed, killing its 96 passengers.
The cause of the crash has been the subject of a fierce debate that has divided Poland ever since, with some believing it was deliberate and others rejecting that as a conspiracy theory.
