Russian police have briefly detained six people who staged a protest in Moscow demanding the authorities disclose the whereabouts of jailed activist Ildar Dadin, who has said he was tortured in prison.
The protesters were released after spending about four hours in custody on January 4.
Prominent opposition politician Aleksei Navalny tweeted two photos of a woman whom he described as a lone protester.
One photo depicts the woman holding a placard reading "Where is Ildar Dadin?" and another condemning "torture in Russian concentration camps."
The second photo shows the woman being escorted away by two policemen.
Dadin, 34, became the first Russian citizen jailed for participating in more than two unsanctioned public gatherings in 180 consecutive days under a controversial 2014 law that critics say is part of a redoubled Kremlin effort to stifle dissent during President Vladimir Putin's third term.
Dadin received a three-year sentence in December 2015, and it was later reduced by six months.
Last month, Russian human rights campaigners draped a banner calling for Dadin's release from a St. Petersburg bridge.