MOSCOW (AP) — Thousands of people took to the streets Sunday across Russia to demand the release of jailed opposition leader Alexei Navalny, keeping up the wave of nationwide protests that have rattled the Kremlin. Hundreds were detained by police.

The authorities mounted a massive effort to stem the tide of demonstrations after tens of thousands of people rallied across the country the previous weekend in the largest and most widespread show of discontent the country has seen in years.

Police so far have detained over 260 participants in protests held in many cities across Russia's 11 time zones, according to the OVD-Info, a group that monitors arrests.

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

The 44-year-old Navalny, an anti-corruption investigator who is the best-known critic of President Vladimir Putin, 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.

Navalny's team 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.

As part of a multipronged effort by the authorities to block the protests, courts have jailed Navalny's associates and activists across the country. His brother Oleg, top aide Lyubov Sobol and three other people were put under two-month house arrest Friday on charges of alleged violations of coronavirus restrictions during last weekend’s protests.

Prosecutors also demanded that social platforms block the calls for joining the protests on the internet.

The Interior Ministry has issued stern warnings to the public not to join the protests, saying participants could be charged with taking part in mass riots, which carries a prison sentence of up to eight years. Those engaging in violence against police could face up to 15 years.

Nearly 4,000 people were reportedly detained at demonstrations on Jan. 23 calling for Navalny’s release took place in more than 100 Russian cities, and some were given fines and jail terms. About 20 were accused of assaulting police and faced criminal charges.

Just after Navalny's arrest, his team released a two-hour video on his YouTube channel about an opulent Black Sea residence purportedly built for Putin. The video has been viewed over 100 million times, helping fuel discontent and inspiring a stream of sarcastic jokes on the internet.

Putin has said that neither he nor any of his close relatives own the property, and on Saturday, construction magnate Arkady Rotenberg, a longtime Putin confidant and his occasional judo sparring partner, claimed he owned the property.

Navalny fell into a coma on Aug. 20 while on a domestic flight from Siberia to Moscow. He was transferred to a Berlin hospital two days later. Labs in Germany, France and Sweden, and tests by the Organization for the Prohibition of Chemical Weapons, established that he was exposed to the Novichok nerve agent. Russian authorities have refused to open a full-fledged criminal inquiry, citing a lack of evidence that he was poisoned.

When he returned to Russia in January, Navalny was jailed for 30 days after Russia’s prison service alleged he had violated the probation terms of his suspended sentence from a 2014 money-laundering conviction that he has rejected as political revenge.

On Thursday, a Moscow court rejected his appeal to be released, and another hearing next week could turn his 3 1/2-year suspended sentence into one he must serve in prison.