The international rights group Amnesty International has named Russian opposition leaders Aleksei Navalny and Sergei Udaltsov "prisoners of conscience."
Navalny, a prominent lawyer and anticorruption blogger, and Udaltsov, a radical activist, were detained on May 6 during mass protests in Moscow and initially ordered to pay a fine.
They were detained again a few days later and sentenced to 15 days in jail on charges of resisting police.
Last month, Amnesty International recognized three members of the all-female opposition punk group Pussy Riot as prisoners of conscience and called for their immediate release.
The women face seven years in jail for performing a song critical of Putin in Moscow's largest cathedral.
Regarding Navalny and Udaltsov, the head of Amnesty International's Russia office, Sergei Nikitin, said the men had "sought to realize their right to freedom of demonstrations" using "nonviolent, peaceful methods," but ended up being jailed by Russian authorities.
The statement came hours after police detained another protest leader as part of a crackdown on demonstrations against President Vladimir Putin's rule.
'Brutal And Barbaric'
Prominent environmental and opposition activist Yevgenia Chirikova was arrested on May 18 at a sit-in on Moscow's Kudrinskaya Square along with a handful of demonstrators.
She told RFE/RL she sustained injuries during her arrest.
Russian opposition activist Yevgenia Chirikova is detained by police Moscow's Kudrinskaya Square.
"There was absolutely no need to detain us in such a brutal and barbaric manner," she said. "My hand was injured during the detention, I can't close my fingers. Another woman who is with us, a physically disabled pensioner, is covered in bruises. Most importantly, the police still haven't stated the reason for our arrest."
Her lawyer says riot police had no right to hold Chirikova, a mother of two young children, overnight.
She was released on May 18 and faces a court hearing on May 21.
The protesters moved to the square on May 16 after riot police uprooted them from another Moscow site where they had been camped for a week.
The sit-in demonstrations in Moscow are part of a so-called "unlimited" protest, which the opposition launched after violent clashes between demonstrators and police on May 6, the day before Putin was sworn in for a third term as Russia's president.
The protesters are seeking to call attention to alleged electoral fraud in last December's parliamentary polls, the March presidential poll, high-level corruption and what they see as the damage Putin's continued rule is inflicting on Russia. Putin has been in power as president or prime minister for the past 12 years.
Police have detained more than 40 demonstrators over the last two days in the Russian capital, including Solidarnost opposition movement leader Ilya Yashin and Petr Verzilov, a member of the dissident art group "Voina."
Yashin was sentenced to 10 days arrest on Thursday, also on charges of disobeying police.
With reporting by Reuters and AFP