RUSSIAN police arrived at the Moscow headquarters of opposition leader Alexei Navalny and broke into the building using an angle grinder, a spokeswoman for Mr Navalny said.
Police broke in and immediately started questioning people, it has been reported.
Photographs posted on the internet by Mr Navalny’s supporters showed about 14 policemen gathered outside his office door.
Police shut down a TV studio at the office which had been broadcasting online news bulletins, but another studio in a different location continued to operate.
They detained six of Navalny’s supporters at the Moscow studio and around 16 protesters in other parts of Russia, OVD-Info, an independent monitoring group, said.
It was unclear where Navalny was, but a group of police officers was stationed near his home. Navalny said he planned to attend the Moscow protest later on Sunday.
Hundreds of supporters of Mr Navalny began a nationwide day of protests against the authorities on Sunday, calling on voters to boycott what they say is a rigged presidential election on March 18.
In a video address to his supporters, he said: “Your life is at stake” as he urged them to attend rallies that have been planned in more than 100 cities.
He said: “How many more years do you want to live with these thieves, bigots and perverts in power? We’ve already endured this for 18 years.”
Putin’s spokesman, Dmitry Peskov, warned that unsanctioned rallies would lead to “certain consequences”.
They claim it is rigged to ensure Putin cements his power and is able to secure his fourth term in office.
Last month Mr Navalny was not allowed to run in the election by Russia’s central election commission due to a suspended prison sentence for fraud, but thousands still turned out to endorse him during protests.
In an interview with CNN, Mr Navalny said: “Putin has been in power for 18 years now.
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