On January 2, a Moscow court sanctioned his detention for two months.
Mokhnatkin is accused of attacking a policeman in Moscow during a Strategy-31 protest on New Year's Eve.
Twenty-eight people were detained during the protest, which was held in support of Article 31 of the Russian Constitution, which guarantees the right of peaceful assembly.
All of those detained were released, except Mokhnatkin.
Mokhnatkin first came to prominence when he was detained during a protest in 2009 for attacking a police officer. He was sentenced to 2 1/2 years in prison.
Amnesty International recognized him as a prisoner of conscience.
He was pardoned by then-President Dmitry Medvedev in April 2012.