MOSCOW — The Moscow police arrested about 400 people, including the country’s main opposition leader, during a street protest on Wednesday against abusive police tactics.

At an event where the outcome, mass arrests, seemed only to confirm the protesters’ complaints, riot police dragged demonstrators from the crowd seemingly at random and arrested news photographers and reporters.

The police said more than 200 protesters had been detained. OVD Info, an independent group monitoring arrests, said more than 400 had been arrested. Among them was Aleksei A. Navalny, the opposition leader.

Organizers had called the protest to support a Russian journalist, Ivan Golunov, whom supporters say the police had framed on drug charges last week. Pictures produced by the police that seemed to show a drug lab in the reporter’s apartment had been faked. In a rare about-face, the authorities released Mr. Golunov on Tuesday, acknowledged that there was no evidence to support the charges and opened an investigation into the police who had detained him.