Russian opposition leader Lyubov Sobol is among scores of people detained during an unauthorised Moscow protest.
Ms Sobol was in a taxi about to set off for the rally when police officers dragged her into a black van, which swiftly sped off.
Protesters are gathering in the Russian capital after authorities disqualified a number of opposition candidates from standing in local elections.
Russian officials said that 30 people had been arrested out of 350 attendees.
But other reports put that number much higher. OVD-Info, a group which runs a hotline for reporting detentions, said that at least 311 people had been arrested, including some journalists, and there were at least six reports of beatings by police.
Officers in riot gear had earlier moved into the capital and warned people not to protest.
Ms 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 Saturday.
Authorities said she was being held for violating regulations for street demonstrations.
In July, Ms Sobol was dragged out of the electoral commission office on a sofa.
Speaking to independent broadcaster Dozhd before her detention, she said the authorities “are doing everything they can to try to intimidate the opposition”.
“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,” she added.
Georgy Alburov, from the anti-corruption group FBK, which has links to Mr Navalny, tweeted from the back of a police vehicle saying he had been arrested at the protest.
Shortly afterwards, Russian officials announced an investigation into FBK for alleged money laundering of a billion rubles ($15.3m, £16.6m) – though it did not name any individuals.
The nation’s investigative committee said that funds has 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 8 September.
Officials said many of the signatures required for their candidacy applications were invalid. But protesters say they were excluded for political reasons.
Despite the recent arrests and a ban on further rallies, demonstrators are expected to take to the streets of Moscow again on Saturday.
Authorities have launched an investigation into the protests.
On Friday, they detained a number of men – including Alexey Minyaylo, an independent politician and aide to Ms Sobol – in connection with the “mass unrest”, a charge which carries a sentence of up to 15 years in jail.
Prominent Kremlin critic Alexei Navalny has also been arrested in connection with the rallies.
He fell ill in jail with a swollen face and rashes over his body, and was briefly hospitalised.
Doctors said he had had an extreme allergic reaction, but Mr Navalny and his personal doctor said he may have been poisoned.