Stalin is still a hero for many Russians for his role in defeating Nazi Germany
Russian delegates have reacted angrily after an OSCE parliamentary session in Lithuania voted for a remembrance day for victims of Nazism and Stalinism.
The parliament of the pan-European security body passed a resolution equating the roles of the USSR and Nazi Germany in starting World War II.
Moscow's delegation tried but failed to have the resolution withdrawn.
Relations between Russia and the OSCE are already strained over hurdles to election observers in Russia itself.
The resolution, meant to mark the 20th anniversary of the fall of the Iron Curtain, said that Nazi Germany and Stalin's Soviet Union brought genocide and crimes against humanity to Europe.
It called for making 23 August a day of remembrance for the victims of Stalinism and Nazism.
On that day in 1939, Germany and the Soviet Union signed a pact that carved up Eastern Europe between the two countries.
Vilija Aleknaite-Abramikiene, the Lithuanian delegate who drafted the resolution, said the intention was not to insult anybody, but to remember those who perished in World War II.
A big majority of the delegates present backed the resolution. Out of 213 members there, from 50 countries, only eight voted against it and four abstained, OSCE parliamentary spokesman Klas Bergman told BBC News.
There were some reports that Russia's delegates left the hall immediately after the motion was passed but Mr Bergman denied this, saying they "continued to participate in the session".
Joseph Stalin continues to be a hero to many Russians, who credit him with defeating Nazi Germany.
But in some countries occupied by the Soviets, and in the Baltic states in particular, the Soviets are reviled as much as the Nazis.