Kira Yarmysh revealed Mr Navalny had been taken to a police station on Saturday morning. But, writing on Twitter, she said reasons for his detention had not been made clear.
It is understood that he has now been transferred to hospital with a suspected broken finger.
He had recently been preparing for demonstrations on 9 September against plans to increase the age at which citizens can claim a pension.
The 42-year-old – who has been jailed repeatedly for organising anti-Kremlin protests – was barred from running in Russia’s presidential election in March under what he says were false pretences.
He called for a nationwide boycott of the vote, which president Vladimir Putin went onto win overwhelmingly.
Mr Navalny was last sentenced to 30 days in prison in May for his role in organising nationwide demonstrations against Mr Putin.
He had called for protests in more than 90 cities and towns against the increasingly autocratic rule of the president. The slogan “Putin is not our tsar” was widely used.
Soon after the 18 March election, Mr Navalny’s team told The Independent it would be focusing on “building agility and strength”, in anticipation of any future opportunity.