Russia's most famous prisoner, Kremlin critic and former oil tycoon Mikhail Khodorkovsky, began life as a free man in Germany on Saturday after his surprise pardon by President Vladimir Putin.

After spending more than 10 years behind bars, Russia's former richest man was quitely escorted from his prison in northwestern Russia on Friday and boarded a plane to Berlin in an operation worked out behind the scenes with the German government.

The lightning speed of his release led some observers to suggest that Khodorkovsky might have flown into forced exile but Putin's spokesman dismissed such suggestions.

"He is free to return to Russia. Absolutely," Peskov told AFP on Saturday. He declined however to say whether any conditions were attached to his release or whether he would be free to participate in politics.

Khodorkovsky, 50, wrote two letters to Putin: one a short legal letter, and the other a longer, personal missive, Peskov added. He declined further details.

Putin had stunned Russia on Thursday by saying his fierce critic had asked for clemency on humanitarian grounds as his mother was ill. "Guided by humanitarian principles," the Russian strongman signed a pardon decree on Friday.

In his first remarks since his release, Khodorkovsky said in a statement on Friday he did ask Putin for a pardon but his request did not amount to an admission of guilt and thanked Hans-Dietrich Genscher, Germany's foreign minister from 1974-1992, for helping negotiate his release.

"I am very much looking forward to the minute when I will be able to embrace my loved ones," the father of four said, without specifying his future plans.

Khodorkovsky's 79-year-old mother Marina, who has cancer, was expected to fly out to Berlin, where she had undergone treatment before, to see her son.

The Russian opposition magazine The New Times, for which Khodorkovsky wrote a column about his prison life, said the former tycoon called the editorial office to express gratitude for support.

"The most important today is Freedom, Freedom, Freedom," the magazine quoted him as saying. "A lot lies ahead, the release of those hostages who still remained in prison, first and foremost Platon Lebedev," he said, referring to his jailed business partner.

Washington urges Russia to go further

The United States welcomed Khodorkovsky's release but called on the Kremlin to go further still.

"We have repeatedly expressed our concerns about due process violations and selective prosecution in Russia, including against Khodorkovsky and his business partner Platon Lebedev, who remains in prison," said Secretary of State John Kerry.

"The United States strongly encourages Russia to pursue reforms that establish a transparent, independent, and reliable judicial system that upholds its commitments to human rights, the rule of law, and non-discrimination."

Khodorkovsky's release coincided with an amnesty for prisoners convicted of non-violent crimes that is expected to free the two jailed members of Pussy Riot band, Nadezhda Tolokonnikova and Maria Alyokhina, just as the Kremlin readies to host the Winter Olympics in February

Thirty foreign and Russian Greenpeace activists, arrested on hooliganism charges after their protest against Arctic oil drilling, are also expected to escape prosecution.

Khodorkovsky said he had made the pardon request to Putin on November 12, catching his defense team and even his family off guard.

"He is doing well under the circumstances," Genscher told German public television.

"Even though I don't understand Russian, I was able to witness in the car his (phone) talk with his relatives, and that was touching, understandably after the 10 years that he had to endure," he said on Friday.

Genscher said he was unaware if Khodorkovsky planned to remain in Germany: "I think that he wants to take a deep breath and wait to take his family in his arms tomorrow."

'Behind the scenes'

Less than three hours after the Kremlin released the pardon decree, Khodorkovsky's lawyers said he had left his prison colony in the town of Segezha in the Karelia region.

Genscher, 86, arranged the flight on a private jet and picked Khodorkovsky up at the airport in Berlin.

Chancellor Angela Merkel hailed the "behind the scenes" work of Genscher, her spokesman Steffen Seibert said.

"He worked successfully on possibilities for a solution with a great level of commitment and the support of the chancellor and the foreign ministry," Seibert said.

From the airport, Khodorkovsky was reportedly taken to Berlin's luxury Adlon Hotel near the Brandenburg Gate from which Genscher was seen leaving in the early evening.

The release drew the curtain on the highest profile criminal case in post-Soviet Russia which has harmed the country's investment climate and become a symbol for the selective persecution of Kremlin enemies under Putin.

"It has not sunk in yet," his mother Marina Khodorkovskaya told state television.

Interviewed on CNN television, Khodorkovsky's son Pavel, said he was in a state of "happy shock" following his father's release, adding that they had spoken on the phone.

"He's doing very well, sounds certainly very good and very happy."

Pavel said he "didn't think" his father would seek political asylum in Germany.

Khodorkovsky had been due to be released in August 2014.

The Kommersant broadsheet, citing unnamed sources, said Khodorkovsky had decided to seek a pardon after a meeting with representatives of Russia's security services, who had told him about the threat of a third case against him and his mother's worsening health.