Police and hundreds of demonstrators squared up in central Moscow at an unauthorised march against police abuse in the wake of the high-profile detention of a Russian journalist.
More than 20 demonstrators have been detained, according to a monitoring group.
The rally comes one day after a stunning police decision to drop charges against Ivan Golunov, who was charged last week with drug dealing.
After a public outburst of protest, including by three of Russia's most respected newspapers, the case was dropped on Monday for lack of evidence.
Police claimed to have found drugs in Mr Golunov's apartment.
The OVD-Info group that monitors political arrests said more than 20 people were detained at the march.
Associated Press journalists saw opposition leader Alexei Navalny being seized.
- Associated Press