Despite bitter cold and intimidation attempts, protests are taking place across Russia.

Thousands of people in Russia’s Far East and Siberia gathered on Saturday in support of jailed opposition leader Aleksei A. Navalny in what turned out to be the largest nationwide showdown in years between Russian authorities and critics of the Kremlin.

In the eastern regions of Russia, a country with eleven time zones, protests began hours before demonstrations in Moscow were due to begin. Soon after dawn in the capital, Saturday appeared to be the biggest day of protest in the country since at least 2017 – although it was not clear whether the contradiction would succeed in persuading the Kremlin to change course.

In the cities of Vladivostok on the Pacific and Irkutsk and Novosibirsk in Siberia, recordings of well over 1,000 people showed chants like “We are responsible here!”. and “We’re not going!”

In Yakutsk, the coldest city in the world, numerous demonstrators defy temperatures of minus 60 Fahrenheit in the icy fog. In Khabarovsk, the city on the Chinese border where protests against the Kremlin took place last summer, hundreds of people returning to the streets faced overwhelming numbers of riot police.

“I have never been a great believer in Navalny, and yet I understand very well that this is a very serious situation,” said Vitaliy Blazhevich, 57, a Russian university professor, in a telephone interview about why he chose Mr. Navalny in Khabarovsk .

“There is always hope that something will change,” said Blazhevich.

Protesters demand Navalny be released from prison, but the Kremlin is holding on.

Aleksei Navalny, a 44-year-old anti-corruption activist who is the most prominent domestic critic of President Vladimir V. Putin of Russia, was poisoned in Siberia in August with a military-grade nerve agent in what Western officials called an assassination attempt by the Russian state.

He was flown to Germany and recovered. And last Sunday after flying home to Moscow, he was arrested at passport control.

Russian authorities say Mr Navalny violated a suspended sentence he received six years ago and are trying to limit him to years in prison. After he was jailed on Monday for an initial 30-day sentence, his supporters called for protests, arguing that only street pressure could avert what they describe as an attempt by Mr Putin to get his favorite opponent out of the way to vacate.

These protests took place across Russia on Saturday, organized in part by Mr Navalny’s extensive network of local offices. Local officials did not approve the protests – citing the coronavirus pandemic, among other things – and threatened to arrest anyone who attended.

The video showed police officers fighting with protesters in Vladivostok and Khabarovsk, but there were no immediate reports of large-scale violence. OVD-Info, an activist group tracking arrests during protests, reported 174 arrests nationwide as of midday in Moscow – a number that would surely increase later in the day.

In the normally quiet town of Yuzhno-Sakhalinsk, a fisheries and energy center on an island north of Japan, hundreds of people took part in the protests on Saturday.

Some schools have postponed classes while one held a basketball tournament on Saturday to keep teenagers away from the protests, said Lyubov Barabashova, a city-based journalist.

The police did not prevent the demonstrators from gathering in front of the regional government headquarters, Ms. Barabashova said. When a police officer announced via megaphone that the rally was illegal, the demonstrators sang in response: “Putin is a thief! Freedom to Navalny! “

The Kremlin has weathered waves of protests in recent years, and there was no immediate indication that this time would be any different. There were growing signals that the Russian government intended to respond to the protests with a new wave of repression.

The US embassy in Moscow warned American citizens to stay away from the protests on Saturday – an announcement that the Channel One news anchor pointed out that the US had indeed organized them.

“This is very important: information on the location and time of the unauthorized events scheduled for tomorrow has been posted on the American embassy website,” said the Channel One host. “As they say, draw your own conclusions.”

The Russian authorities said they had opened criminal investigations against protest organizers. And on Friday, the main evening newscast on Russian state-controlled Channel One devoted about a third of the show to Mr. Navalny – a clear departure from the typical state news media practice of ignoring him.

Russia is trying to prevent young people from taking to the streets.

A ninth grader in the Russian city of Yekaterinburg asked his classmates this week why they didn’t like President Vladimir V. Putin.

According to their teacher Irina V. Skachkova, citing the imprisoned opposition leader Aleksei A. Navalny, they replied: “Putin has a palace that was built with stolen money, and Putin is a thief himself.”

Mr Navalny’s dramatic return to Russia from Germany on Sunday and his immediate arrest, followed by the release of a video documenting Putin’s alleged secret palace on the Black Sea, have captured many young Russians and prompted authorities to make an effort to keep them away from protests.

Some universities threatened to expel students if they were caught in the protests for the release of Mr Navalny, which are being organized in dozens of cities across Russia, even though local officials did not authorize them.

The Ministry of Education urged families to spend the weekend doing non-political activities such as “a walk in a park or a forest”.

Russia’s telecommunications regulator said it had ordered social networks to cut posts for Saturday’s protests and the country’s top investigative agency has opened a criminal investigation into alleged inciting minors to join.

In the days leading up to the protests on Saturday, Aleksei A. Navalny’s team published a comprehensive investigation describing a secret palace built for President Vladimir V. Putin on the Black Sea.

The report, released Tuesday, less than 24 hours after Mr Navalny was arrested, was the latest blow in the Russian opposition leader’s dramatic battle against Mr Putin.

The investigation – including floor plans, financial details, and interiors of a site that Mr Navalny said cost more than $ 1 billion – appeared to provide the most comprehensive record of any huge residence that the president allegedly kept for himself has built southern Russia’s green coast.

The Kremlin denied the findings of the report, which went online as a 113-minute YouTube video and illustrated text version, urging users to post pictures of Putin’s alleged luxury on Facebook and Instagram. The video has been viewed more than 65 million times on YouTube.

“They will steal more and more until they bankrupt the whole country,” says Navalny in the video, referring to Putin and his circle. “Russia sells huge amounts of oil, gas, metals, fertilizer and wood – but people’s incomes are falling and falling because Putin has his palace.”

Few people had heard of the nerve agent Novichok until 2018, when Western officials accused Russia of using him in the UK attempt on a former spy. It made headlines in September when Germany said the poison had made Russian dissident Aleksei A. Navalny sick.

But scientists, spies, and chemical weapons specialists have known and feared Novichok for decades. It is a powerful neurotoxin that was developed in the Soviet Union and Russia in the 1980s and 1990s and can be delivered as a liquid, powder, or aerosol. It is said to be more deadly than nerve substances better known in the West. like VX and Sarin.

The poison causes muscle spasms that can stop the heart, buildup of fluid in the lungs that can also be fatal, and can damage other organs and nerve cells. Russia has made several versions of novichok, and experts say it is unclear how many times they have been used, as the resulting deaths can seem like nothing more sinister than a heart attack.

Such could have been the case of Sergei V. Skripal, a former Russian spy who lives in Salisbury, England. When Mr Skripal was barely conscious in a park in March 2018, there was no obvious reason to suspect poisoning – other than that his daughter who was visiting him had the same symptoms.

British intelligence agencies identified the substance as novichok and accused Russia. The attack turned into a major international scandal that further shook relations between Moscow and the West. The British identified Russian agents who they said had flown to the UK, applied the poison on the door handle of Mr Skripal’s house and left the country, leaving a trail of videos and chemical evidence.

The government of President Vladimir V. Putin has consistently denied any involvement and has put forward a number of alternative theories. And just months before the Salisbury attack, Putin said Russia had destroyed all of its chemical weapons.

Ivan Nechepurenko and Richard Pérez-Peña contributed to the coverage.