Hundreds of his supporters have begun a nationwide day of protests against the authorities
Russian police have reportedly forced their way into the Moscow office of the opposition leader Alexei Navalny (Pron: Al-ex-ee Nav-AL-knee) and started questioning people.
Navalny supporters claim the police said they were looking into reports there was a bomb in the office.
Hundreds of his supporters have begun a nationwide day of protests against the authorities, they want voters to boycott what they believe will be a rigged presidential election on 18 March.
Mr Navalny was banned from running in the election by Russia's central election commission because of a suspended prison sentence for fraud last month.
But thousands of his supporters turned out during demonstrations across the country to endorse him.
Despite his best efforts, it's unlikely Mr Navalny would win even if he was allowed to run for presidency.
Polls show Vladimir Putin is on course for a comfortable re-election which would keep him in office for another six years.