Russian prosecutors on Monday backed a request to imprison opposition leader Alexei Navalny for several years on old embezzlement charges, after police detained a record number of anti-Kremlin protesters across the country.

On Sunday, demonstrators defied government warnings and rallied across the country -- including in Moscow and Saint Petersburg where authorities enforced unprecedented lockdowns of the city centre -- in a second weekend of mass protests over the arrest of President Vladimir Putin's most prominent opponent.

Alexei Navalny faces years of jail time in several different criminal cases, despite calls from Western governments for his release
Alexei Navalny faces years of jail time in several different criminal cases, despite calls from Western governments for his release Navalny team Youtube page / Handout

The protests -- stretching from the Pacific port of Vladivostok to the northwestern city of Pskov -- came ahead of a high-profile court hearing that could see Navalny imprisoned for several years.

Navalny's detention and the crackdown on protesters has sparked an outcry in the West.

EU foreign policy chief Josep Borrell is expected to press the Kremlin to free Navalny when he heads to Moscow on Thursday.

Main developments surrounding the poisoning of Russian opposition campaigner Alexei Navalny.
Main developments surrounding the poisoning of Russian opposition campaigner Alexei Navalny. AFP / Gal ROMA

Navalny, 44, is facing charges of violating the terms of a 2014 suspended sentence for embezzlement and could be jailed for two-and-a-half years.

The General Prosecutor's Office said in a statement on Monday that it backed a request by the prison service to change the suspended sentence to jail time.

"This motion is considered lawful and justified," the statement said.

Police detain pro-Navalny protesters in Saint-Petersburg
Police detain pro-Navalny protesters in Saint-Petersburg AFPTV / Alexander ZHDANOV

Navalny was detained at a Moscow airport in mid-January after flying back to Russia from Germany where he was recovering from an August poisoning.

He blames the attack with the Novichok nerve agent on Putin and the FSB security agency.

Navalny was detained shortly after returning from Germany where he was treated for poisoning with the Novichok nerve agent
Navalny was detained shortly after returning from Germany where he was treated for poisoning with the Novichok nerve agent AFP / PETRAS MALUKAS

The anti-corruption campaigner is being held in a high-security detention centre and faces years of jail time in several different criminal cases, despite calls from Western governments for his release.

Over 5,000 people were said to have been detained at Sunday's protests
Over 5,000 people were said to have been detained at Sunday's protests AFP / Vasily MAXIMOV

Navalny's team has urged his sympathisers to gather in front of the Moscow City Court on Tuesday to show support for the opposition politician, who is expected to appear in person, according to a court spokeswoman.

In recent years, Navalny has served a number of brief jail stints but never a long prison term.

On Sunday, thousands of people rallied across Russia to demand freedom for Navalny and changes to Russia's tightly-controlled political system.

Law enforcement officers blocked protesters during a rally in support of jailed opposition leader Alexei Navalny in Saint Petersburg
Law enforcement officers blocked protesters during a rally in support of jailed opposition leader Alexei Navalny in Saint Petersburg AFP / Olga MALTSEVA

OVD Info, which monitors arrests at opposition protests, said more than 5,400 people had been detained -- a record in the history of modern Russia.

Ahead of Sunday's demonstration, authorities locked down the centre of the capital, with hundreds of police lining the streets, central metro stations closed and restrictions on the movements of pedestrians.

Protesters and monitors pointed to the police's increasingly harsh tactics, saying they used batons and tasers on peaceful demonstrators, and some were badly beaten.

In Saint Petersburg, a policeman was seen aiming his service weapon at the protesters, while in Vladivostok police used non-lethal weapons against a demonstrator, according to OVD Info.

"Political repression is ramping up," Leonid Drabkin, top OVD Info coordinator, told AFP.

Putin's spokesman Dmitry Peskov on Monday defended the clampdown.

"We are talking about unlawful rallies," he told reporters. "Naturally, police take measures against participants of these unlawful rallies -- hence the number of detained."

But the Saint Petersburg ombudsman said authorities unleashed "essentially a military operation" against protesters and paralysed the city.

"Batons and tasers will not solve existing problems. Violence only breeds intransigence and engenders bitterness," ombudsman Alexander Shishlov said in a statement.

"This is a path towards great trouble. It's time to stop."

Kremlin critics say a concerted effort is under way to silence Navalny's team, dismantle his Anti-Corruption Foundation and pressure his family.

On Monday, a court fined Navalny's wife Yulia 20,000 rubles ($265) for participating in an "unauthorised" rally in her husband's support.

Ahead of Sunday's demonstrations, Navalny's brother Oleg and several key allies including prominent activist Lyubov Sobol were placed under house arrest until March 23 for allegedly violating coronavirus restrictions and calling on people to join protests.

On Monday, France urged Germany to scrap the Nordstream II gas pipeline project with Russia in protest of Navalny's detention.