Vladimir Kara-Murza was released on Thursday - in what was the largest prisoner of war exchange between Russia and the West since the Cold War - and spoke to his wife and children on the phone.
"Hello, Dad? It's so good to hear from you," said the activist's daughter when she heard his voice on the phone from the Oval Office, accompanied by President Joe Biden, her mother and brother.
"Words can't express this. I was sure I would die in prison. I can't believe what's happening. I think I'm still sleeping in my cell in Omsk and not hearing your voice. But I just wanted you to know that you've done a marvellous thing in saving so many people. There are few things more important than saving human lives," said Kara-Murza, to the emotional looks and smiles of her family.
Meanwhile, Biden welcomed her, emphasising that "there is no greater job than protecting and bringing American citizens home".
Kara-Murza's friends also reacted to the release with a statement, quoted by the Russian press: "Today [Thursday] marks the end of 843 nightmarish days for Russian-British citizen and opposition leader Kara-Murza, his family and close friends," it read.
Kara-Murza was convicted of treason and other charges in 2023 and sentenced to 25 years in prison.
Who is Vladimir Kara-Murza?
A dual Russian citizen and prominent opposition politician, he was arrested in 2022 after criticising the war in Ukraine that had begun weeks earlier.
A columnist for The Washington Post, Kara-Murza, 42, was awarded the Pulitzer Prize this year.
He fell ill in 2015 and 2017 due to two near-fatal poisonings that he blames on the Kremlin. His wife and his lawyers claim that his health is deteriorating in prison as a result of the poisonings.
