MOSCOW (AP) — Thousands of people took to the streets Sunday across Russia's vast expanse to demand the release of jailed opposition leader Alexei Navalny, keeping up a wave of nationwide protests that have rattled the Kremlin. Over 750 were detained by police, according to a monitoring group.

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

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.

On Sunday, police detained more than 750 people in protests held in cities across Russia's 11 time zones, according to the OVD-Info, a group that monitors arrests.

In the far eastern port of Vladivostok alone, more than 100 people were detained after protesters danced on the ice and rallied in the city center.

The city of Novosibirsk in eastern Siberia saw one of the biggest rallies, with thousands marching across the city chanting “Putin, thief!” in 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. More than 80 protesters were detained.

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. After police cordoned off the area around the square, the protest shifted to another central square a mile away. Police deployed in force at that location too, detaining scores and putting them into police buses.

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

Prosecutors also demanded that social media platforms block calls to join the protests.

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

Russia has seen extensive corruption during Putin's time in office even as many ordinary citizens struggle financially.

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, claiming 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 Navalny's 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.