Moscow police detained dozens of people at a protest against "political repressions" near the headquarters of the Russian security service, including a well-known human rights activist.
The protest demanding to free political prisoners was called by supporters of men convicted this year on terrorism charges, many of whom said in court they had been tortured by the FSB security service.
Mostly young protesters turned up to participate in picketing near the FSB building, with some unfurling signs calling the security service the "Federal Service of Lawlessness" while others shouted "Shame!"
The OVD Info website, which tracks detentions at political protests, listed 49 people as detained by police, adding that some were hurt in the altercations.
An AFP correspondent at the scene saw about 30 detentions.
Some at the protest spoke out against the planned constitutional reforms currently in process of being adopted, which will give President Vladimir Putin a chance to run for two more terms.
"The authorities are using targeted repressions in order to rule forever," said leftist politician Sergei Udaltsov.
"We are seeing an attempt to overthrow the constitutional regime, to reset the (presidential) term limits, solidarity is more important than ever," he told AFP prior to getting arrested.
Putin on Saturday signed the constitutional reform law, but it still needs to be backed by the constitutional court as well as a simple majority in a public vote announced for April 22.
Veteran human rights campaigner Lev Ponomaryov, who chairs the For Human Rights organization, also was among those arrested in the sweep by national guard and police, which announced via loudspeaker the action is not authorized.
He was seen arguing with police that pickets don't require authorisation, before being led to a police bus.
OVD Info later said that Ponomaryov was beaten at the police station by masked officers, posting a photo of the 78-year-old campaigner sporting what looked like a bruise on his face.
A court in Penza region last month handed terms between six and 18 years to seven young men who were accused of creating a terrorist organization called "Network" with the goal of overthrowing the government.
They all denied charges in court, and most said they had been tortured in custody with electrodes and beatings to extract confessions since their arrests in 2017 and 2018.
Two other defendants in the case are still on trial in St. Petersburg.