Russian opposition leader Alexei Navalny was released from jail Friday after serving 25 days for organizing a wave of protests.

"We will, of course, not bend under any pressure," Navalny said after he returned to the office of his Fund for Fighting Corruption.

Navalny was arrested June 12 as thousands of protesters gathered for an unsanctioned demonstration in Moscow. Protests in response to Navalny's call occurred in more than 100 Russian cities and towns that day, one of the strongest showings by the marginalized opposition for several years.

Nearly 2,000 people were detained at the protests, most of them in Moscow and St. Petersburg. Navalny was sentenced to 30 days in jail, but a court later reduced that to 25.

Navalny told supporters at his headquarters that he was glad that "work continued even in my absence, and I will continue working on it in the same cheerful manner. We will, of course, not bend under any pressure."

Navalny has been jailed several times in connection with protests and convicted in a fraud case widely seen as politically motivated.

The anti-corruption campaigner, who is the most persistent thorn in the Kremlin's side, aims to run for Russian president in 2018.