Chanting slogans against President Vladimir Putin, tens of thousands took to the streets Sunday across Russia to demand the release of jailed opposition leader Alexei Navalny, keeping up nationwide protests that have rattled the Kremlin. More than 3,800 people were detained by police, according to a monitoring group, and some were beaten.

Russian authorities mounted a massive effort to stem the tide of demonstrations after tens of thousands rallied across the country last weekend in the largest, most widespread show of discontent that Russia has seen in years. Yet despite threats of jail terms, warnings to social media groups and tight police cordons, the protests again engulfed cities across Russia’s 11 time zones on Sunday.

The 44-year-old Navalny, an anti-corruption investigator who is Putin’s best-known critic, was arrested on Jan.17 upon returning from Germany, where he spent five months recovering from nerve-agent poisoning that he blames on the Kremlin. Russian authorities have rejected the accusations. He was arrested for allegedly violating his parole conditions by not reporting for meetings with law enforcement when he was recuperating in Germany.

The United States urged Russia to release Navalny and criticized the crackdown on protests.

“The US condemns the persistent use of harsh tactics against peaceful protesters and journalists by Russian authorities for a second week straight,” US Secretary of State Antony Blinken said on Twitter.

The Russian Foreign Ministry rejected Blinken’s call as a “crude interference in Russia’s internal affairs” and accused Washington of trying to destabilize the situation in the country by backing the protests.

On Sunday, police detained more than 3,800 people at protests in cities nationwide, according to OVD-Info, a group that monitors political arrests.

In Moscow, authorities introduced unprecedented security measures in the city center, closing subway stations near the Kremlin, cutting bus traffic and ordering restaurants and stores to stay closed.

Navalny’s team initially called for Sunday’s protest to be held on Moscow’s Lubyanka Square, home to the main headquarters of the Federal Security Service, which Navalny claims was responsible for his poisoning. Facing police cordons around the square, the protest shifted to other central squares and streets.

Police were randomly picking up people and putting them into police buses, but thousands of protesters marched across the city center for hours, chanting “Putin, resign!” and Putin, thief!” — a reference to an opulent Black Sea estate reportedly built for the Russian leader that was featured in a widely popular video released by Navalny’s team.

“I’m not afraid, because we are the majority,” said Leonid Martynov, who took part in the protest. “We mustn’t be scared by clubs because the truth is on our side.”

At one point, crowds of demonstrators walked toward the Matrosskaya Tishina prison where Navalny is being held. They were met by phalanxes of riot police who pushed the march back and chased protesters through courtyards, detaining scores and beating some with clubs. Still, demonstrators continued to march around the Russian capital, zigzagging around police cordons.