At least 11 activists have been detained on Moscow's Red Square as they were reading Russia's constitution out loud, a group that monitors protest activity says.
Among the activists detained on May 12 was Ildar Dadin, according to OVD-Info, a website that tracks the detention of activists.
Dadin was the only person in Russia to have served prison time for contravening a tough law clamping down on protests in the country.
The activists were taken to a nearby police station, OVD-Info said. It remains unclear whether they could face charges.
"A police officer approached us and warned us that if we don't leave, they will detain us," Dadin was quoted by TASS news agency as saying. "Ten minutes later police detained us."
Lawyer Aleksei Lipster told RIA Novosti news agency that Dadin was still in police custody but that officers had not written up a report.
Dadin, 35, held repeated peaceful demonstrations against Russian President Vladimir Putin's rule without official permission.
He was convicted and sentenced to prison in December 2015 under a controversial law that criminalizes participation in more than one unsanctioned protest in a 180-day period.
He was released on February 26 after the Supreme Court threw out his conviction and ordered the case closed.
Dadin's plight attracted additional attention last autumn, when he alleged he and other inmates had been tortured and abused at a prison in Karelia.