The US and UK have condemned the "harsh tactics" used against protesters and journalists in Russia - as more than 2,500 people have reportedly been arrested at rallies in support of the jailed Putin critic Alexei Navalny.
"We call on Russian authorities to release all those detained for exercising their universal rights," said US State Department spokesman Ned Price on Saturday.
Britain's Foreign Secretary Dominic Raab also condemned "the Russian authorities' use of violence against peaceful protesters and journalists" and called on the government to "release citizens detained during peaceful demonstrations".
Mr Navalny's wife, Yulia Navalnaya was briefly held at a rally in Moscow - the location for one of up to 70 marches this weekend - where more than 500 people have been detained, according to independent reports.
She has now been released, according to Russian agency Interfax.
The gatherings, which police have declared illegal, are the first by Mr Navalny's supporters since he was arrested last weekend on his return to Moscow, after spending five months in Germany recovering from novichok poisoning
In response to the arrests at the rallies, the British government has urged Russia to respect international human rights commitments.
"We are deeply concerned by the detention of peaceful protesters and continue to monitor the situation closely," the Uk's foreign ministry said in a statement.
Roughly 2,250 people have been detained across Russia because of the protests, according to monitoring group OVD-INFO which says it is the largest number of detentions the organisation has ever recorded.
Monitors from Amnesty International decried "the viciousness of the police response" to protests against Mr Navalny's "politically-motivated detention".
It comes after thousands took part in rallies and marches in Yekaterinburg, Irkutsk, Izhevsk, Nizhny Novgorod, Arkhangelsk and other cities in Russia's east, where, it is claimed, protests are the biggest for some years.
Among these was one in Yakutsk, where people gathered to demonstrate in -50C temperatures.
And thousands more took to the streets of the capital Moscow and St Petersburg, where images showed huge crowds moving though the streets and OVD reported 200 detentions.
Videos from upwards of a dozen cities showed scuffles and people being dragged away by police, while others were hit with batons - including some in Moscow - or left injured on the ground.
One group of Mr Navalny's supporters reached the Matrosskaya Tishina prison where the opposition figure is being held. Some were detained by the police.
One tweeted video, apparently filmed in Moscow, is said to show an ally of Mr Navalny, the lawyer Lyubov Sobol, being grabbed by police and dragged away while taking part in a TV interview.
Ms Sobol was later charged with repeat violation of the rules on protesting, according to Interfax.
The Sky team in Pushkinskaya Square, Moscow, says they saw at least 50 people carried away, apparently randomly.
At least 40,000 gathered in the capital, Reuters estimated. Authorities claimed some 4,000 people showed up.
Sky producer Anastasia Leonova says many in the crowd are shouting "Navalny", "Putin should leave" and "Shame on you".
She says people of all ages are taking part, including a lot of students and school children, with the oldest probably in their 70s.
Many of those driving past, including those in new or expensive cars, are beeping their horns in solidarity with those protesting on the streets, Leonova adds.
One man, Vina Lapkis, a 30-year-old Russian who has lived in US, told Sky News: "We're out here because we love our country and we hate the illegal ridiculous stuff that's going on here.
"We don't agree with killing people, we don't agree with arresting people just because you don't like their political views. We don't agree with using police as a force to fight your own people and we hope that if we fight long enough we're going to win some day."
Another, Moscow lawyer Bilal Khotkin, added: "In Russia, we don't have any possibility to show the Government what we want. We want change, because Putin is more than 20 years and it's a big period. We have big corruption, Putin is head of corruption.
"I want to show world we maybe have a lot of problems. I'm here with this crowd and I want to show the world we want to change the situation."
There have also been reports that mobile phone and internet services in Russia have suffered outages as police crack down on anti-Kremlin protesters.
Authorities sometimes interfere with communication networks to make it harder for protesters to get in touch with each other and the wider world online.
Dozens of journalists have been held across Russia, according to reports, and protests are also taking place outside Russia in Sweden, Finland, the UK and other countries.
Mr Navalny, 44, who is one of President Vladimir Putin's most outspoken critics, blames Moscow for the attack that nearly killed him, although the Kremlin denies any involvement.
He is charged with breaking his bail conditions - and is facing a potential three-and-a half-year jail term if found guilty.
Anyone who takes part faces charges of rioting, fines, problems at work, prison and even threats over child custody as the Russian state tries to crack down on the demonstrations, which could be the largest against Mr Putin since 2018.
Officials also enforced a crackdown in the run-up to the demonstrations, arresting members of Mr Navalny's team, including his spokeswoman Kira Yarmysh.
They launched an investigation after young Navalny supporters flooded TikTok with anti-Putin videos, pushing for people to support the action this weekend and using the using the hashtags #freenavalny and #23Jan.
The content has been viewed more than 300 million times.
Anger mounted against Mr Putin this week after Mr Navalny's team released a documentary exposing a vast and opulent palace built by Russia's leader on the Black Sea coast.
The programme claims the complex - 39 times larger than Monaco - cost £1bn to build and was funded through illicit money.
It is said to have a casino, an underground ice hockey complex and a vineyard.
More than 60 million people have now viewed the Russian-language video on YouTube within three days of it being published.
On Friday, ahead of the weekend of planned protests, Mr Navalny issued a statement saying he wanted it known that he had no plans to take his own life in prison.
The arrest of Mr Navalny has attracted widespread criticism from Western leaders, sparking new tensions in the already strained relationship with the US.
Despite the plans for the protests, Mr Putin's grip on power appears solid, with the 68-year-old regularly recording approval ratings of more than 60%, many times higher than those of Mr Navalny.
'Our kids are being brainwashed'
Eyewitness by Diana Magnay, Moscow correspondent
The rally is not due to start until 2pm, but already here in Moscow, the police are making arrests and there are several hundred people around waiting.
It reminds me very much of the protests in the summer of 2019. There are huge numbers of press following each arrest. I haven't seen any beatings yet, but the arrests are not pleasant.
Among those attending are Olga and Vladislav Sheglov, father and daughter.
Mr Sheglov told me: "I came here because I cannot live like this anymore, what they're doing is not acceptable.
"I always tell myself we have the best country, but the worst government."
His daughter Olga said: "Our kids are being brainwashed. You have families with low income and they have another view of politics.
"When we saw the Putin's palace investigation, we were so shocked. We used to vote for him, but this was the last straw. We believe 150%, a million percent that Navalny was poisoned."
Another person at the protest, 16-year-old Yaroslavl, who we are not naming fully because he's 16, said: "There'll probably be more detentions than normal because it's such a big day.
"I'm a bit concerned, but so many people have come together to defend their own opinion and to defend Russia.
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"I was told at school not to come, that they might have extra lessons today, but I ignored them. And my parents were even more serious about me not coming, but I ignored them too."
He said that today everyone went out not for Navalny, but for themselves, to fight for their rights.
