Russia's Vladimir Putin and Poland's Donald Tusk attended the remembrance for more than 20,000 Polish prisoners of war who were ordered shot by Josef Stalin in the eastern Russian town of Katyn in 1940, Poland.pl reported.
It marked the first official Russian recognition of what Poles claim was genocide, the report said.
It wasn't until 1990 that Soviet leader Mikhail Gorbachev publicly admitted the NKVD state police that evolved into the KGB had carried out the killings, Radio Free Europe said.
Prior to that, Moscow had maintained the slaughter was the work of German Nazis.
Despite the apparent diplomatic warming, Moscow didn't invite Polish President Lech Kaczynski to the remembrance ceremony, which RFE said was likely in response to his anti-Russian stance. Kaczynski will host a separate ceremony Saturday in Katyn, the report said.