While he was being questioned, Navalny said investigators accompanied by armed police officers raided the office of his corruption-fighting fund.
Navalny, a vehement foe of President Vladimir Putin, was convicted in late December of fraud and given a 3½-year suspended sentence. The court said that until his appeals were exhausted he must remain under house arrest.
He has been allowed to leave his home in recent days, but has been followed by law enforcement officers posted outside his door.
Navalny posted on Twitter that he was met Friday morning by 12 police officers and taken to the federal Investigative Committee, which had summoned him for questioning about his anti-corruption activities.
He said the whole thing was like a movie: "Three police are with me in the car and three in plainclothes in the car next to us. Too bad I didn't bring my dark glasses."
A few hours later, Navalny tweeted that he had emerged from the interrogation to learn that his fund was being raided. The director of his fund, Roman Rubanov, wrote on Twitter that all of their computers had been seized.
Navalny's lawyer, Olga Mikhailova, said he had refused to answer the investigators' questions because he believed that their aim was to bring criminal charges against him, the Tass news agency reported.
The night before, police detained 13 people from about 100 at a protest in central Moscow in support of Navalny and his brother, who was sent to prison in the same case. The protesters were confronted by a much larger group of pro-Kremlin activists, which led to scuffles.
In St. Petersburg, about 300 people rallied peacefully in support of Navalny.
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