More than 600 Russian protesters have been detained over an unauthorized rally in Moscow, amid reports of police violence.
Demonstrators had gathered in Moscow after authorities disqualified a number of opposition candidates from standing in local elections.
Leading activist Lyubov Sobol was arrested before she could reach the protest, attended by 1,500 people.
Video from the demonstration shows officers using their batons against demonstrators while making arrests.
Russian officials initially said there had been just 30 arrests and 350 attendees.
Monitoring group OVD-Info, which runs a hotline for reporting detentions, had been keeping a running toll, which rapidly jumped from a few dozen arrests to several hundred.
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Officers in riot gear had earlier moved into Moscow and warned people not to protest. Russian news agency TASS reported that one police officer had been injured while making an arrest.
However, protesters also reported mistreatment at the hands of police.
Footage broadcast on Russian TV and shared on social media showed police pin people to the ground, kicking or using batons on them.
Alexander Svidersky, a member of a district electoral commission, said he was arrested while out with his dog, which he managed to pass off to an acquaintance before being bundled into a police van. OVD-Info reported he said he was later taken out and hit around the kidneys before being dragged to another van.
Detainees at one police station also told OVD-Info they were threatened with having their fingers “cut off” if they did not allow their fingerprints to be taken.
Lyubov Sobol, a lawyer and video blogger, is one of the candidates excluded from the local elections. She has been on hunger strike for 21 days, and called on others to join the unsanctioned protest on August 3.
She was in a taxi about to set off for the rally when police officers dragged her into a black van.
Hours after Lyubov Sobol’s arrest, she tweeted from a police station, saying she had spent three hours being driven “all over Moscow” by a dozen masked officers.
Authorities said she was being held for violating regulations for street demonstrations.
Last month, Lyubov Sobol was dragged out of the electoral commission office on a sofa.
Speaking to independent broadcaster Dozhd before her detention, Lyubov Sobol said the authorities “are doing everything they can to try to intimidate the opposition”.
She said: “That is why it is important to come out today to show that Muscovites are not afraid of provocation and they are ready to continue to stand up for their rights.”
Shortly afterwards, Russian officials announced an investigation into FBK for alleged money laundering of a billion roubles ($15.3 millio) – though it did not name any individuals.
The nation’s investigative committee said that funds had been knowingly obtained through criminal means.
Authorities detained more than 1,000 demonstrators last weekend during a demonstration, one of the biggest crackdowns in years.
Election authorities have barred opposition candidates from taking part in Moscow city authority elections planned for September 8.
According to officials, many of the signatures required for their candidacy applications were invalid. But protesters say they were excluded for political reasons.
Another protest held in solidarity in St Petersburg had some 1,000 attendees – but it had not been banned by local officials, and there are no reports of arrests.