Members of the punk provocateur band Pussy Riot were among 18 protesters detained in Moscow after spending the night near the Red Square.
Pussy Riot member Maria Alekhina said on Twitter that she and other protesters who stayed the night near the famous Moscow landmark were detained by police early Wednesday.
Activists at the OVD Info group said 18 people were taken to a nearby police station.
A few thousand people rallied outside the Red Square on Tuesday in the boldest anti-government demonstration in years to protest against the court conviction of opposition leader Alexei Navalny and his brother.
Police allowed the protesters to stay for about two hours before dispersing the crowd.
OVD Info said more than 200 people were also arrested on Tuesday although many have been released.