Journalist and Putin critic Masha Gessen said she was among 24 people being taken to police stations. Moscow police said about 20 people were detained.
The State Duma, or lower house of parliament, was expected to pass the bill later in the day, ignoring Western criticism that it curbs basic freedoms and concerns among activists that it is fuelling hate attacks on homosexuals.
Anti-gay activists were seen singing Orthodox Christian prayers and crossing themselves.
On May 25th, nearly 30 demonstrators were arrested at an unsanctioned rally against the bill:
Watch video from the scene:
The bill will now proceed to the Federation Council and must be signed by President Vladimir Putin.