Ilya Yashin, a Russian opposition activist freed from jail in Thursday's prisoner swap, pledged to carry on his political fight against President Vladimir Putin from abroad, but expressed fury at having been deported against his will.

The prisoner swap, the largest since the Cold War, saw eight Russians, including a convicted murderer, exchanged for 16 prisoners in Russian and Belarusian jails, many of them dissidents. It was hailed as a win by Western leaders who feared for the dissidents' lives after the death in jail last year of politician Alexei Navalny.

But Yashin, imprisoned in 2022 for criticising Putin's full-scale invasion of Ukraine, said he had not given his consent to deportation and that others in more urgent need of medical care should have gone instead of him.

"From my first day behind bars I said I was not willing to be a part of any exchanges," he said in an emotional news conference in Bonn on Friday during which he occasionally removed his glasses to blink back tears.

He directed his ire not at the Western governments that had secured his release, who he said had faced a difficult moral dilemma, but at the Kremlin for expelling a political rival against his will.

"What happened on Aug. 1 I don't view as a prisoner swap ... but as my illegal expulsion from Russia against my will, and I say sincerely, more than anything I want now to go back home," he added.

He was speaking alongside activists Vladimir Kara-Murza and Andrei Pivovarov at the freed prisoners' first public appearance since arriving in Germany.

On their second day out of prison, where they had had limited contact with the outside world, Kara-Murza and Yashin especially seemed fired with resolve, and to have kept abreast of world events. All expressed scorn for the government of Putin whom Kara-Murza described as an illegitimate usurper. Yashin pledged to continue his work "for Russia" from abroad. "Though I don't yet know how," he added.

Pivovarov agreed: "We will do everything to make our country free and democratic, and get all political prisoners released."

Former Russian president Dmitry Medvedev, commenting on the prisoner exchange on Thursday, said that what he called traitors to his country should rot and die in prison, but that it was more useful for Moscow to get its own people home.

Kara-Murza recounted that when he had been asked by prison officers to sign an appeal for clemency, he had taken the pen offered and written "that I consider him (Putin) not to be a legitimate president, to be a dictator, a usurper and a murderer."

Kara-Murza blamed Putin for the deaths of Navalny and Russian politician Boris Nemtsov, killed in Moscow in 2015, as well as thousands of Ukrainians, including children killed in the bombing of a Kyiv hospital last month.

Kara-Murza had been serving a 25-year sentence and said he had been certain he would never see his wife again and would die in a Russian jail.

While he said he was glad to be free, he also expressed reservations about the manner of his leaving, which he called an illegal expulsion under the letter of Russian laws. He also acknowledged the dilemma German Chancellor Olaf Scholz had faced in deciding whether to release convicted murderer Vadim Krasikov to secure their safety.

The operation was "about saving lives, not exchanging prisoners," he said. "Scholz is being criticised in some quarters for the difficult decision to release Putin's personal killer... But easy decisions come only in dictatorships."

Had things been easier, Navalny might not have died, he added.

"It's hard for me not to think that, maybe if these processes had somehow moved quicker ... if there had been less resistance that the Scholz government had to overcome in terms of freeing Krasikov, then maybe Alexei would have been here and free," he said.

He described an ordeal that had amounted to psychological torture. A prison doctor had told him he had just a year to a year-and-a-half of life remaining as a consequence of two poisonings he had suffered.

He was allowed to speak with his wife just once and his children twice in more than two years of imprisonment, he said, and spent 10 months in solitary confinement. A Christian, he was banned from attending church, he added.