Twenty opponents of Russian intervention in Ukraine were arrested after they released doves, a symbol of peace, into the air.
It happened outside the defence ministry in Moscow.
Russian President Vladimir Putin has said Russia reserves the right to to protect its compatriots in the majority ethnically Russian Crimea region of Ukraine.
Putin denies Russian forces were directly involved in the bloodless seizure of Crimea.
He says the uniformed troops, who bare no national insignia, are "local self-defence forces."
Russian navy ships have reportedly blockaded the strait that separates the Ukrainian Black Sea peninsula from Russia.