Russia has opened criminal cases that could put protesters in jail for years after tens of thousands of people took to the streets at the weekend to demand the release of opposition leader Alexei Navalny.
The crackdown came as supporters of Navalny, who is facing more than a decade behind bars on charges seen as politically motivated, defied the Kremlin with calls for further rallies.
In the biggest demonstrations the country has seen in years more than 3000 people were arrested nationwide on Saturday, a record for Russian protests. Investigators said they were opening criminal probes over alleged hooliganism and violence used against the police, which could hand some of those who attended the rallies sentences of up to five years in prison.
Police said 4000 people turned out in Moscow but observers and media put that figure in the tens of thousands. More than 10,000 took to the streets in St Petersburg and there were demonstrations in dozens of other cities across the country, including in Yakutsk, Siberia where temperatures reached -50C.
The rallies were called by Navalny, Russia's most prominent critic of Vladimir Putin, after he was arrested on his return to Moscow following months in Germany recovering from a poisoning he says the Kremlin orchestrated.
Riot police charged demonstrators on Pushkin Square in central Moscow, using batons on the crowd. Several bloodied demonstrators were dragged off the square, while authorities said some 30 people were taken to hospital.
Investigators said they were looking at "a large amount of photo and video material" that showed violence against police by protesters. A widely shared clip showed officers being pelted by snowballs, while demonstrators attacked what appeared to be a FSB car in another video.
However, in St Petersburg, prosecutors said they would look into police violations after a video showed officers pushing over and kicking a middle-aged woman. The hospital where she was taken reported that she was in a "serious condition" with a head injury.
The US and UK have condemned the response from Russian police, while Josep Borrell, the EU's foreign policy chief, said the bloc would discuss "next steps" today. Andrzej Duda, the Polish president, told the Financial Times that he supported further sanctions.
Moscow, meanwhile, said it would summon representatives of the US embassy over allegedly promoting the protests with a safety warning on its website. The Kremlin, which has consistently sought to play down Navalny's political significance, said "few people" had protested compared with the "many" who voted for Putin.
"If you compare the numbers, you will understand how few these people are," said the President's spokesman Dmitry Peskov.
Leonid Volkov, who runs Navalny's network of offices across Russia, announced further mass protests at the weekend and called for a greater turnout. "Anyone who doesn't come out is considered (by Peskov) to be pro-Putin!" he wrote on Twitter.
Navalny says his poisoning was carried out by the FSB on the orders of Putin, a charge the Kremlin denies. Multiple laboratory tests in Europe showed he was poisoned with a chemical from the same Novichok group used against former Russian agent Sergei Skripal in Salisbury.
Putin has denied the Russian state was behind the attack, despite a sting by the opposition leader last year in which an FSB agent apparently confirmed details of the operation during a recorded phone call.
Some protesters said they had mixed feelings about the opposition leader but were on the streets because of wider anger over Putin's 20-year rule.