It was the first time the two countries' leaders had marked Katyn together
The Russian and Polish prime ministers have together marked the 1940 Katyn massacre of 22,000 Polish troops, in an unprecedented move.
Russia's Vladimir Putin had invited Poland's Donald Tusk to the ceremony commemorating the massacre.
The Soviet secret police was responsible, but for half a century the Soviet Union blamed it on the Nazis.
Both leaders spoke of reconciliation at Wednesday's ceremony, though their statements appeared qualified.
"A lie was told for decades, but we cannot blame the Russian people for it," said Mr Putin, who stopped short of apologising for the killings.
"Inhuman totalitarianism" had been to blame for the "martyred death of both Soviet citizens and Polish officers," he said, in comments reported by AFP news agency.
'Word of truth'
Mr Tusk said the two countries "still had a way to go on the road to reconciliation".
"A word of truth can mobilise two peoples looking for the road to reconciliation," he said.
"Are we capable of transforming a lie into reconciliation? We must believe we can."
The April 1940 killings were carried out by the NKVD Soviet secret police on the orders of Soviet dictator Joseph Stalin.
They shot members of the Polish elite - officers, politicians and artists - in the back of the head and dumped their bodies in mass graves.
It was only in 1990 that Soviet leader Mikhail Gorbachev admitted the USSR was responsible.
The issue has acted like an open sore on Polish-Russian relations, the BBC's Adam Easton reports from the Polish capital, Warsaw.
Russia has refused to open its archives to Warsaw and Russian commentators sometimes still peddle the lie about German guilt, he says.
But the joint ceremony is seen as representing a recent improvement in relations.
"However hard it may be, we must try to ... come to terms with a common historical truth and realise that we cannot go on living in the past alone," said Mr Putin.
The killings took place at various sites, but the western Russian forest of Katyn has become their chief symbol.