Updated 3.15pm

A Russian court has sentenced respected Gulag historian Yury Dmitriyev to more than three years in prison in a controversial sexual abuse case, his lawyer said.

Dmitriyev, head of the prominent rights group Memorial in Karelia in northwestern Russia, spent decades locating and exhuming mass graves of people killed under Soviet dictator Joseph Stalin's rule. 

Memorial said the prosecution of the researcher, who was tried behind closed doors, is part of a growing crackdown on dissenters.

Defence lawyer Viktor Anufriyev told reporters outside the courthouse in the  northwestern city of Petrozavodsk that the judge found his client guilty of sexual assault against his adopted daughter and sentenced him to "three years and six months in a strict-regime penal colony".

The lawyer said he had not yet received the written verdict and it was possible that with time served in pre-trial detention Dmitriyev, 64, could be free in September.

He had been facing up to 15 years under the charges.

Anufriyev said the historian had been cleared of pornography charges that stem from his first criminal case.

The researcher was first arrested in late 2016 on child pornography charges, then acquitted in 2018, and then arrested again in a new sexual assault case.

The prosecution claimed that the historian sexually abused his adopted daughter, charges he denies.

"There are no doubts that Yury Dmitriyev is innocent," Memorial said ahead of the verdict.

"The truth about the past does not fit the state's historical narrative," Memorial added. 

Supporters including prominent advocates at home and abroad say the case against Dmitriyev is an attempt to punish the historian who has called attention to one of the darkest chapters in Russia's history.

Dmitriyev is known for helping open the Sandarmokh memorial in a pine forest in Karelia in memory of thousands of victims -- including many foreigners -- murdered in 1937 and 1938.