Russian opposition leader Alexei Navalny walked free on Friday after 25 days in jail for organising unauthorised protests, his press secretary said.

"Today at 9:00 am (0600 GMT) Alexei Navalny was taken from the cell block to the Moscow central district police department and they have now let him out of there," his spokeswoman Kira Yarmysh wrote on Twitter.

The 41-year-old lawyer was sentenced to 25 days in police cells for organising a day of protests against President Vladimir Putin's rule and corrupt officials on June 12 which saw more than 1,700 protesters detained nationwide.

A crowd of some 50 journalists and supporters as well as four members of pro-Kremlin groups were waiting on Friday outside the detention centre where Navalny had been held.

After his release, Navalny went to the office of his anti-corruption foundation, greeting smiling supporters, photographs posted on Twitter showed.

Navalny has declared his aim to stand for president in elections in 2018 and eradicate the country's rampant official corruption. He is already travelling around the country promoting his bid.

Electoral authorities say however that a suspended prison sentence makes him ineligible to run for office.

Navalny has suffered a wave of attacks and legal obstacles since he announced his bid for the presidential race in December.

The campaign has not yet begun and Putin has not confirmed whether he will seek re-election.

Navalny is serving a five-year suspended sentence for embezzlement in a case he had condemned as politically motivated. The Central Electoral Commission, which organises the vote, said last month that this makes him ineligible to stand for office.

On Thursday, police blocked off Navalny's Moscow campaign office and seized computers and campaign materials in a probe into alleged rent violations.

During the raid, a volunteer named as Alexander Turovsky was beaten by police and suffered a head injury and concussion, a spokeswoman for the Moscow campaign office told Interfax news agency.

Turovsky was hospitalised but then discharged and is now in court, facing a charge of resisting police, spokeswoman Yelena Slesareva said.

Navalny's campaign has seen him suffer eye damage after a protester threw green dye in his face in April.

The opposition leader was last put behind bars for 15 days in March after thousands of people flooded into central Moscow for an unauthorised anti-corruption demonstration.

He commented after his latest detention that another jail stint would allow him to sleep and play backgammon.