Police detained more than 2,700 people and broke up rallies in Moscow and across Russia on Sunday as supporters of Kremlin critic Alexei Navalny braved the bitter cold and a massive show of police force to demand he be set free.

The nationwide rallies are the second straight weekend of protests that are part of a high-risk opposition campaign to try to pressure the Kremlin into freeing President Vladimir Putin’s most prominent opponent.

The opposition politician was arrested on Jan. 17 after returning to Moscow from Germany where he had been recovering from a nerve agent poisoning in Russia last summer. He accuses Putin of ordering his murder, which the Kremlin denies.

Navalny’s dramatic return to Moscow despite the obvious threat of arrest and the protests that his jailing has sparked pose a major challenge for Putin who has dominated the Russian political landscape for over two decades.

In a highly unusual move, police imposed a security lockdown in the heart of the capital on Sunday, sealing off streets to pedestrians near the Kremlin, closing metro stations and deploying hundreds of riot police as snow fell.

Yulia, a 40-year-old protester in Moscow, said she had joined the rallies despite having a panic attack the night before because of worrying about repercussions for taking part.

-I understand that I live in a totally lawless state. In a police state, with no independent courts. In a country ruled by corruption. I would like to live differently- she said.

Police have said the protests were illegal as they had not been authorised and would be broken up. Authorities have said demonstrators could spread COVID-19.

At one point a column of protesters marched towards the prison in northern Moscow where Navalny is being held, chanting “Let him go!”

Police put turnout at the Moscow protest at around 2,000 people. Reuters reporters estimated crowds of several thousand that were smaller than last weekend when police estimated 4,000 people and the opposition put the figure at 50,000, an assertion the authorities dismissed as nowhere near the truth.

Police detained at least 2,737 people nationwide, including 681 in Moscow, according to OVD-Info, a protest monitoring group. The figures were expected to rise.

Yulia Navalnaya, the Kremlin critic’s wife, was among those detained.

-If we stay quiet, then they could come for any of us tomorrow- she wrote on Instagram before joining the protests.

The protest is a test of Navalny’s support after many of his prominent allies were targeted in a crackdown this week. Several, including his brother Oleg, are under house arrest.

In the far eastern city of Vladivostok, video footage showed protesters chanting “Putin is a thief” as they linked hands and marched in temperatures of around -13 Celsius (8.6 Fahrenheit). Police detained more than 100 people in the city, OVD-Info said.

In Tomsk, the Siberian city that Navalny visited before suddenly collapsing on a domestic flight last August, demonstrators gathered in front of a concert hall and chanted “Let him go!”.

Navalny, 44, is accused of parole violations which he says are trumped up. A court is due to meet next week to consider handing him a jail term of up to three and a half years.

The West has told Moscow to let Navalny go and his allies have appealed to U.S. President Joe Biden to impose sanctions on 35 people who they say are Putin’s close allies.

Seeking to galvanise supporters at home, Navalny put out an online video this month that has been viewed over 100 million times, accusing Putin of being the ultimate owner of a sumptuous Black Sea palace. The Kremlin leader has denied this.

On the eve of the protests, Arkady Rotenberg, a businessman and Putin’s former judo sparring partner, said he owns the property.