Police detained more than 450 people at rallies in Siberia and Russia’s Far East on Sunday as supporters of Kremlin critic Alexei Navalny took to the streets to protest his jailing, despite biting cold and the threat of arrest.

The rallies, also set to take place in Moscow and other cities later on Sunday, follow large protests last weekend and are part of a campaign 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.

Police have said the protests have not been authorised and will be broken up, as they were last weekend. Over 4,000 people were detained at those rallies, according to OVD-Info, a protest monitoring group.

In the far eastern city of Vladivostok, where a protest began at 0200 GMT, police prevented protesters from accessing the centre, forcing them to relocate to the waterfront and the frozen waters of the Amur Bay.

Video footage showed protesters chanting “Putin is a thief” as they linked hands and marched on the ice in temperatures of around -13 Celsius (8.6 Fahrenheit).

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!” and held up Russian flags.

OVD-Info said police had so far detained 465 people, including 108 in Vladivostok.

Dozens of people in the east Siberian city of Yakutsk turned out in temperatures of -42 C (-44 F).

“This is the first time I’ve come to a protest. I’m just fed up with the total lawlessness of the authorities,” said Ivan, a protester who declined to give his surname.

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.

“If we stay quiet, then they could come for any of us tomorrow,” Yulia Navalnaya, the Kremlin critic’s wife, wrote on Instagram.