Members of the US Congress have called for the immediate release of Russian opposition figure Vladimir Kara-Murza as they marked the second anniversary of his imprisonment.

“The bottom line is that Vladimir Kara-Murza will not be forgotten,” Democratic Senator Ben Cardin, chairman of the Senate Foreign Relations Committee, said at an event on Capitol Hill. “We are going to work to set him free and to set Russia free.”

Dual Russian-British citizen Mr Kara-Murza, a journalist and opposition activist, was jailed in April 2022 and convicted of treason last year for denouncing the war in Ukraine.

He is serving 25 years, the stiffest sentence handed down to a Kremlin critic in modern Russia. He is among a growing number of dissidents held in increasingly severe conditions under President Vladimir Putin’s political crackdown.

Sen Cardin was joined by a bipartisan group of members from both the House and Senate, seeking to increase the pressure not only on Russian authorities to force the release of Mr Kara-Murza, but to ensure that the Biden administration continues working to force his release as the US has done in previous cases of Russian political prisoners.

Sen Jim Risch, the top Republican on the committee, renewed his call on Tuesday for Secretary of State Antony Blinken to designate Mr Kara-Murza as a “wrongfully detained person”, an appointment that would help elevate his case and provide resources to his family in America as they fight for his release.

The charges against him stem from a March 2022 speech to the Arizona House of Representatives in which he was critical of Russia’s full-scale invasion of Ukraine.

Mr Kara-Murza, who twice survived poisonings that he blamed on Russian authorities, has rejected the charges against him as punishment for standing up to Mr Putin and likened the proceedings to the show trials under Soviet dictator Josef Stalin.

Mr Kara-Murza’s wife, Evgenia Kara-Murza, who lives in the US with their three children, joined politicians at the event in pleading for her husband’s release.

“I want to thank each and every one of you here for joining with me in my fight, not just for Vladimir’s freedom, but truly for his life,” she told the crowd.

Ms Kara-Murza has said her husband has spent months in solitary confinement, a punishment that has become common for Kremlin critics and is widely viewed as designed to put additional pressure on them.

Most recently, Mr Kara-Murza had been held in a penal colony in the Omsk region, though his supporters said in late January that he apparently was no longer there.