Claims that Joseph Stalin ordered thousands of Polish officers executed without trial at Katyn in 1940 represent a violation of the Russian Constitution and an slander against the Soviet leader, a lawyer for Stalin’s grandson Yevgeny Dzhugashvili said on Tuesday, according to RIA Novosti.

More than 20,000 Polish officers, police and civilians imprisoned during the 1939 partitioning of Poland by the Soviet Union and Nazi Germany were executed in Katyn, near the western Russian city of Smolensk.

“The accusation of a criminal offense that is not based on a court verdict - this is obviously a prejudicial statement,” the lawyer, Leonid Jura, said.

A court in Moscow began hearings on a libel suit filed by Dzhugashvili against the parliament's lower house (State Duma). It approved a resolution on Katyn that blames the massacre on Stalin and other Soviet leaders. A hearing has been set for February 14.

Dzhugashvili earlier filed a lawsuit seeking 100 million rubles ($3.2 million) in damages from Duma deputies, but the court dismissed the case.

The Soviet Union always blamed the Katyn massacre on the Nazis, saying the killings took place in 1941, when the territory was in German hands. However, Soviet leader Mikhail Gorbachev formally admitted in 1990 that the executions took place around 1940, and were carried out by the NKVD.

In the 1990s, Russia handed over to Poland copies of documents from top-secret File No.1, which placed the blame squarely on the Soviet Union. In November last year, the lower house of Russia's parliament approved a declaration recognizing the Katyn massacre as a crime committed by Joseph Stalin's regime.

In October 2011, Dzhugashvili filed a defamation lawsuit against Channel One television host Vladimir Pozner, who maintains that Stalin authorized the killing of thousands of the Polish POWs in Katyn.

Dzhugashvili has lost several similar lawsuits filed against other Russian media outlets.