Georgy Malets didn't make it to an anti-Kremlin rally last month. He was detained on his way there by police using facial recognition technology in the Moscow metro.
The 30-year-old Russian photo blogger said the police told him he had been identified by a "Face-ID" camera system and must accompany them to a police station for checks.
"I could see they had some kind of photographs, but they weren't from cameras - it was definitely a photograph from my profile on social networks," Malets told Reuters.
He said he was questioned for four hours as a witness in a criminal case the police said had been opened into earlier rallies, so missed the Jan. 31 protest.
Moscow police and the IT and transport departments of the mayor's office did not respond to requests for comment.
But reports from other protesters who have attend rallies in recent weeks in support of jailed opposition politician Alexei Navalny also indicate police are using facial recognition technology to make preventive arrests and detentions.
An unnamed law enforcement source told TASS news agency the technology used images stored on a database of regular protesters. Rights advocates expressed concern surveillance systems were being used for this purpose.
Samariddin Radzhabov, who was tried for throwing a plastic bottle at a police officer in 2019, said he was also detained on the metro before the Jan. 31 rally and then released.
Ekaterina Schulmann said her husband Mikhail was detained after entering the Moscow metro following a protest.
"They (the police) said he was stopped because 'face control had reacted' and that he had to go to an investigator as a witness in some kind of criminal case," she told Reuters.
Three others at the police station he was taken to had also been identified using facial recognition technology, she said.
The Moscow mayor's office announced it was rolling out a facial recognition system in the metro to spot wanted criminals in 2018, when Russia hosted the soccer World Cup. There are now surveillance cameras all over Moscow.
"There is still a lot we don't know about the facial recognition system in Moscow," said Kirill Koroteev, a lawyer at human rights group Agora.
He said it was unclear how automated the system was, whether all cameras used it and which databases it used.
"At first they said the system would be used to find lost kids and fugitive convicts," said Sarkis Darbinyan, an Internet freedom advocate. "Then they used it to monitor self-isolation during the pandemic, and now, as expected, to monitor protests and activists."
Tens of thousands have protested since Navalny was arrested after returning from Germany following treatment for poisoning in Siberia with what Western countries say was a nerve agent.
He was jailed on Feb. 2 for violating parole on what he says was a trumped-up charge. The Kremlin denies his treatment is politically motivated and questions whether he was poisoned.
The Kremlin has also defended thepolice against accusations of using disproportionate force, saying the protests are illegal because they have not been approved and could spread COVID-19.
Protesters have for several years used social media to coordinate rallies. Now they also have a smartphone app available which, if they are detained, allows them to obtain a lawyer as easily as ordering a taxi, says its founder, Kaloy Akhilgov.
The Advocall app contains a panic button to summon a lawyer, contacts for lawyers and instructions on how to behave if detained or questioned.
Akhilgov, who runs a law firm, said his lawyers would work without charge to help protesters if necessary. About 4,600 people had downloaded the app and over 300 requests for legal assistance had been received via the app, he said.
(Additional reporting by Anastasia Adasheva; Writing by Tom Balmforth; Editing by Andrew Osborn and Timothy Heritage)