MINSK, Belarus — A day before high-stakes talks in Russia, President Aleksandr G. Lukashenko, the embattled strongman leader of Belarus, deployed his security forces in large numbers to deter ongoing protests. But tens of thousands of people still took to the street on Sunday to once again clamor for his resignation.

Despite a huge show of force by riot police officers, masked men and what looked like soldiers, crowds of protesters gathered in Minsk, the capital, and in several other cities, many of them waving the former national flag — a red and white banner that Mr. Lukashenko scrapped soon after coming to power in 1994.

The authorities, in an effort to stop protesters from coordinating movements, also ordered telephone operators to cut mobile internet services, a tactic used during an early round of protests over the disputed presidential election on Aug. 9.

The carnival atmosphere of previous Sunday protests was replaced by tension and fear as police officers revived some of the heavy-handed violence seen when people first took to the streets after the election, which Mr. Lukashenko claimed to have won by a landslide. More than 400 were arrested, the police said. Many said they were beaten at the time of detention.

Credit…Tut.by, via Reuters

As happened in August, the aggressive response by security forces only intensified anger at Mr. Lukashenko, who travels to the Russian town of Sochi on Monday to meet President Vladimir V. Putin of Russia, his main backer.

“The moment has come when it is no longer moral to be neutral,” said Svetlana M. Kopylova, 40, an economist who participated in Sunday’s protest. “I don’t understand how in the 21st century, in the center of Europe, the police can behave this way.”

The identity of many of Mr. Lukashenko’s enforcers has become increasingly difficult to determine. Most wear masks or balaclavas and many wear no insignia. Some of the worst examples of the recent brutality have come from men in civilian clothes. In Minsk, some security units carried assault rifles. In Brest, a city on the border with Poland, the police used water cannons to disperse a crowd.

Throughout the day in Minsk, unmarked vans without license plates raced around the city filled with men who tried to prevent scattered groups of protesters from forming into a single column. But tens of thousands of people eventually managed to unite in the early evening, marching into the center of Minsk from Drozdy, an elite residential area where Mr. Lukashenko and many of his trusted associates have homes.

At a time when nearly all active opposition leaders have been forced to flee the country or been put behind bars by Mr. Lukashenko’s robust security apparatus, the crowd still seemed to be nearly as large as on the four previous Sundays.

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The protests on Sunday dashed Mr. Lukashenko’s apparent hope of being able to silence his opponents before critical talks on Monday with Mr. Putin. The Kremlin, announcing what will be the first meeting between the two leaders since the crisis in Belarus began, said on Friday that the talks would focus on developing the “strategic partnership and alliance” between Russia and Belarus.

Though the protesters are not generally hostile toward Russia, many have said they worry that Mr. Lukashenko will press the Russian leader to support an all-out violent assault on their movement. Mr. Putin announced late last month that, at Mr. Lukashenko’s request, he had formed a “reserve force” of Russian security officers to be deployed in Belarus “if the situation gets out of control.”

One sign at Sunday’s protest read: “Putin We Can Sort This Out By Ourselves.”

In its flagship weekly news show on Sunday evening, Russian state television ignored the day’s large opposition protests in Belarus and instead featured a tiny march led by supporters of Mr. Lukashenko. It portrayed his main rival in the August election, Svetlana Tikhanovskaya, who fled Belarus under duress, as a tool of the Polish government..

Mr. Lukashenko has for years resisted Kremlin efforts to integrate his country more closely with Russia. But his standoff with protesters has made him increasingly reliant on Russian support. .

Credit…Tut.by, via Reuters

The United States and the European Union have both condemned the police violence, questioned the election results and announced plans to impose new sanctions on Belarus.

Mr. Lukashenko, a former collective farm manager, was previously seen as an erratic and highly eccentric partner by many officials in Moscow. He has sought, with some success, to rally the Kremlin to his side by casting the protest movement as a subversive Western plot targeting both countries.

Most of his prominent opponents have been forced to flee from Belarus to Lithuania, Poland and Ukraine. Those who refused to leave have been jailed, including Maria Kolsenikova, who avoided expulsion to Ukraine last week by tearing up her passport at the border.

But the Kremlin is also wary of intervening too forcefully in support of Mr. Lukashenko, for fear of stirring up anti-Russian sentiment. Unlike protesters in Ukraine, who toppled their own president in 2014, Belarusians nearly all speak Russian and harbor no deep-seated hostility toward Moscow.

Credit…EPA, via Shutterstock

“I like Russia, we are very close, but I don’t want to be a Russian citizen,” said Vladislav, an investment banker who refused to give his last name for fear of arrest. “We value what Russia does for us, but we identified ourselves as a separate nation.”

Others warned against severing ties with Moscow.

“Only immature youth believe that we can live without Russia,” said Elizaveta Kalinichenko, a construction engineer. “We are a small country, we cannot get anywhere without it.”

Mrs. Kalinichenko, 69, said she had supported Mr. Lukashenko up until 2015. She even reported her daughter, an opposition activist, to the police, she said. But Mr. Lukashenko’s erratic behavior toward Russia and his disdain toward the Belarusian people made her change her mind.

“He did a lot for Belarus,” she said. But “we need a new, intelligent president now, not a collective farm manager.”

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