Police detained over 1,000 people across Russia on Saturday and used force to break up rallies around the country as tens of thousands of protesters demanded the release of Kremlin critic Alexei Navalny, whose wife was among those detained.
Navalny had called on his supporters to protest after being arrested last weekend as he returned to Russia from Germany for the first time since being poisoned with a nerve agent he says was applied to his underpants by state security agents in August.
The authorities had warned people to stay away from Saturday's protests, saying they risked catching COVID-19 as well as prosecution and possible jail time for attending an unauthorised event.
But protesters defied the ban and bitter cold, and turned out in force.
In central Moscow, where Reuters reporters estimated at least 40,000 people had gathered in one of the biggest unauthorised rallies for years, police were seen roughly detaining people, bundling them into nearby vans.
The authorities said just some 4,000 people had shown up.
Some protesters chanted "Putin is a thief", and "Disgrace" and "Freedom to Navalny!"
Navalny's wife Yulia said on social media she had been detained at the rally. Navalny's mother Ludmila was also at the protest.
Some of Navalny's political allies were detained in the days before the protest; others on the day itself.
The OVD-Info protest monitor group said that at least 1,090 people, including 300 in Moscow and 162 in St Petersburg, had been detained across Russia, a number likely to rise.
It reported arrests at rallies in nearly 70 towns and cities.
Navalny, a 44-year-old lawyer, is in a Moscow prison pending the outcome of four legal matters he describes as trumped up. He accuses President Vladimir Putin of ordering his attempted murder. Putin has dismissed that, alleging Navalny is part of a U.S.-backed dirty tricks campaign to discredit him.
One Moscow protester, Sergei Radchenko, 53, said: "I'm tired of being afraid. I haven't just turned up for myself and Navalny, but for my son because there is no future in this country."
He added that he was frightened but felt strongly about what he called an out of control judicial system.
There was no immediate comment from the Kremlin, which had previously called the protests illegal and the work of "provocateurs".
Video footage from the Russian far east showed riot police in Vladivostok chasing a group of protesters down the street, while demonstrators in Khabarovsk, braving temperatures of around -14 Celsius (7 degrees Fahrenheit), chanted "Bandits!"
Police in Siberia's Yakutsk, one of the coldest cities in the world, where the temperature was -52 Celsius on Saturday, grabbed a protester by his arms and legs and dragged him into a van, video footage showed.
Opposition politician Dmitry Gudkov said the scale of the protests in the regions was unusual.
"Everyone must be really fed up with the stealing and lies if the regions have risen up like this," he wrote on Twitter.
Mobile phone and internet services suffered outages on Saturday, the monitoring site downdetector.ru showed, a tactic sometimes used by authorities to make it harder for protesters to communicate among themselves and share video footage online.
In a push to galvanise support ahead of the protests, Navalny's team released a video about an opulent palace on the Black Sea they alleged belonged to Putin, something the Kremlin denied. As of Saturday the clip had been viewed more than 68 million times.
Navalny's allies hope to tap into what polls say are pent-up frustrations among the public over years of falling wages and economic fallout from the pandemic.
But Putin's grip on power looks unassailable for now and the 68-year-old president regularly records an approval rating of over 60%, much higher than that of Navalny.