The police said they have detained about 70 people for participating in an unauthorized rally in central Moscow in support of the defendants in the so-called Bolotnaya case, seven of whom were handed prison sentences earlier Monday.
OVD Info, a website that documents cases against political prisoners, said that the number of detainees is closer to 240.
Police estimated that about 300 people had gathered on Manezh Square near the Kremlin, but a reporter from Dozhd television said the figure was somewhere between 1,000 to 2,000, Interfax reported.
Opposition leader Alexei Navalny and Pussy Riot members Nadezhda Tolokonnikova and Maria Alyokhina were among those detained on Manezh Square.
Those three enjoyed the dubious pleasure of being inside a police van twice in one day, having been detained earlier on Monday outside the Zamoskvoretsky District Court, where sentences were meted out to eight people convicted of participating in riots and attacking police during a rally at Moscow's Bolotnaya Ploshchad on May 6, 2012.
Seven of the defendants were handed prison terms ranging from 2 ½ to four years and an eighth was given a suspended sentence.
Human rights activists have said that the case is part of an attempt by the Kremlin to clamp down on opposition rallies.