Former tycoon Mikhail Khodorkovsky has been released from jail following a pardon from Russian President Vladimir Putin, his lawyers say.
Mr Putin signed a decree earlier pardoning Khodorkovsky on the basis of “the principles of humanity”.
The president said on Thursday that the former oil magnate had asked him for clemency because his mother was ill.
Khodorkovsky – in custody for a decade – was jailed for tax evasion and theft after funding opposition parties.
The pardon comes after Russian MPs backed a wide-ranging amnesty for at least 20,000 prisoners.
Analysts say Mr Putin may be trying to ease international criticism of Russia’s human rights record ahead of February’s Winter Olympics in Sochi.
A document published by the Kremlin on Friday said the decree would come into force from the day of its signing.
His lawyer Vadim Klyuvgant said prison officials had confirmed that Khodorkovsky, 50, had left the penal colony where he was being held in the Karelia region of north-western Russia.
The former tycoon had eaten lunch at the prison in Segezha as normal on Friday while his release papers were being drawn up, Russian news website Lenta.ru reports quoted an official as saying.
Khodorkovsky was jailed after being convicted of stealing oil and laundering money in 2010. He had been in prison since 2003 when he was arrested and later convicted on charges of tax evasion. He was due to be released next August.
Mr Putin’s announcement on Thursday that his former political opponent had asked for a pardon came as a surprise to commentators.
In a statement on his website, Khodorkovsky’s lawyers said they could not comment on whether a request had been made.
The former head of the now defunct oil giant Yukos, who was once Russia’s richest man, had repeatedly said he would not ask Mr Putin for a pardon because it would be tantamount to admitting guilt.
The Kommersant newspaper, citing unnamed sources, said on Friday Khodorkovsky had made the decision to seek a pardon following the threat of a third trial against him.
“I still don’t know anything. I’m getting everything from the media. Right now I’m watching it on TV,” Interfax quoted Khodorkovsky’s mother, Marina, as saying.
She had earlier told BBC Russian she did not know about any clemency request by her son.
The BBC’s Oleg Boldyrev says many people in Russia doubt that Khodorkovsky was released due to his mother’s ill health.
He says it remains to be seen whether the president’s former rival will pose a political threat after he is freed.
Many Russians who used to be against him as a tycoon now see him as an intellectual forces in the opposition, our correspondent adds.
The amnesty passed in the State Duma on Wednesday covers at least 20,000 prisoners, including minors, disabled people, veterans, pregnant women and mothers.
Mr Putin confirmed it would apply to the two members of punk band Pussy Riot still in prison and Greenpeace activists detained for their protest at a Russian oil rig in the Arctic.