He is courted by American and European diplomats, applauded by a media machine dedicated to vilifying his critics and still has four years left in a presidential term secured last year with a landslide re-election victory.

But President Aleksandar Vucic, Serbia’s strongman leader for more than a decade, never looked so lost as when he appeared this week in an official video on the vast rooftop terrace of his presidential offices to share a bowl of cherries with two lieutenants — and gripe about street protesters calling them rude names, including “abnormal lunatics, murderers and criminals.”

Over-the-top insults, a regular feature of Rottweiler tabloids loyal to Mr. Vucic and pro-government television stations, used to be directed mostly at the president’s enemies, at least in public. But, after weeks of street protests set off last month by two mass shootings, Mr. Vucic is now on the receiving end — and on the defensive like never before since establishing himself in 2012 as the pivot around which Serbian politics turns.

The protests, with calls for the dismissal of senior law enforcement officials and the withdrawal of broadcasting licenses from two pro-government television stations, have grown into a wider revolt against a “climate of violence” blamed on Mr. Vucic and his media attack dogs.

Tens of thousands of people again marched through central Belgrade on Friday evening, blowing whistles and chanting “Vucic go away.” A group of students at the head of the procession waved cardboard figures depicting Mr. Vucic as well as his prime minister, a tabloid editor and a television owner dressed in striped prison uniforms.

It was the sixth in a series of weekly demonstrations organized under the slogan “Serbia against violence.”

“I’m not betting on his downfall because leaders like Vucic have very powerful techniques for survival,” said Vuk Vuksanovic of the Belgrade Centre for Security Policy, an independent research institute. “But there is an open wound and sharks are circling in the water.”

Fishing eagerly in these troubled waters has been Russia, whose ambassador to Serbia, Alexander Botsan-Kharchenko, this week blamed the West for stirring up the protest movement, which has coincided with a flare-up of tensions in Kosovo — the former Serbian territory that declared independence in 2008.

As tens of thousands of people took to the streets of Belgrade late last month, Mr. Vucic ordered his military to move toward northern Kosovo, which is inhabited largely by ethnic Serbs. That move followed a decision by Kosovo’s prime minister, Albin Kurti, to seize municipal buildings in the area and install new ethnic Albanian mayors who won after all but a dozen ethnic Serb voters boycotted the vote.

That infuriated Kosovo Serbs, who attacked NATO peacekeepers, injuring dozens of them and prompting the alliance to rush hundreds of extra troops to northern Kosovo.

Citing the Kosovo troubles, Mr. Botsan-Kharchenko, Moscow’s ambassador, told RT Balkan, a Russian state media operation, that “the goal of the West is to change the politics of Serbia” — a message that played to nationalist Serbs who see Russia as their defender and loathe the United States because of NATO’s 1999 bombing campaign during a war over Kosovo.

A recent survey by Demostat, a research group, found that only 3 percent of Serbs said they admired the United States and wanted good relations with it, in contrast to 22 percent who felt that way about Russia. At the same time, 32 percent favored the European Union and Scandinavian countries, indicating that support for Russia, though strong, lags behind that for the West overall.

And what the Russian ambassador presented as a Western plot to stir up trouble in Kosovo and unseat Mr. Vucic is seen as exactly the opposite by most experts and also by protesters.

Kosovo, said Cedomir Cupic, a political science professor at the University of Belgrade, “is already lost” because there is no realistic possibility of Serbia taking back and ruling more than a million restive ethnic Albanians. But for Russia, he said, the domestic passions it still generates are a godsend for Moscow — a “toothpick that it can always poke around to make the U.S. and Europe feel nervous.”

The violence in Kosovo has also provided a rare piece of good news for Mr. Vucic by playing to his strong suit as defender of Serb interests as he struggles to defuse the street protests.

The situation has exasperated the United States and European Union, which have long tried to lower the temperature and mediate a settlement over Kosovo. They condemned in unusually strong terms the deployment of security forces to northern Kosovo by Mr. Kurti, the Kosovar prime minister.

Tensions in Kosovo “only help Vucic” by revving up passions over territory that most Serbs view as part of their country, said Milomir Mandic, the general manager of Demostat.

“Kurti is constantly helping Vucic,” said Pavle Grbovic, an opposition member of the Serbian parliament from the Free Citizens Movement, which has helped organize the weekly street protests in Belgrade.

“Nobody on the Serbian political scene has done more for Serbia’s position in Kosovo and for Mr. Vucic than Mr. Kurti,” he said.

While senior American and European diplomats have fumed over what they see as provocation by Kosovo, Serbia has reveled in being treated as an important partner.

Gen. Daniel R. Hokanson, a member of the Joint Chiefs of Staff, visited Belgrade this week and hailed what he described as a “truly fantastic partnership” with Serbia.

His praise is part of American efforts to move Serbia away from Russia and toward the European Union. There is little sign that the European bloc is interested in reviving Serbia’s long-stalled application to join and Serbia has balked at imposing sanctions on Russia over the Ukraine war.

But Serbia did vote to condemn Moscow at the United Nations and, to Russia’s fury, Serbian-made weapons have made their way into the hands of Ukrainian forces.

Milovan Drecun, a member of parliament from Mr. Vucic’s party and chairman of the legislature’s Kosovo Committee, said Serbia had made a clear choice to be part of Europe. “Russia belongs to the East and we belong to the West” but Serbia “still needs Russia” because it opposes any recognition of Kosovo as an independent state by the United Nations.

The United States, despite its recent criticism of Kosovo, he said, “is still 100 percent behind Kosovo” claims to statehood “but needs good relations with Serbia because we are the most important country in the Balkans.”

Any lift Mr. Vucic has received from the fallout over Kosovo has been dwarfed by a groundswell of domestic opposition.

The size of the protests — and Mr. Vucic’s failure to mobilize as many people for his own pro-government rally on May 26 — has united the usually fractious opposition in revulsion at back-to-back massacres in early May, one by a 13-year-old shooter at a high-end Belgrade school, the second by a 21-year-old in villages near the capital.

“Maybe I’m too optimistic but I think Vucic is finished,” said Dragan Bjelogrlic, one of Serbia’s best-known actors and a participant in the protests. “He won’t formally lose power right away but the most important thing for all autocrats is not to show they are scared.” Mr. Vucic, he added, “is now looking very scared and this is the beginning of the end.”

Mr. Vucic initially denounced protesters as “scum” and “vultures,” and his media machine ramped up vicious attacks on his opponents. On the eve of a big protest on May 19, Informer, a pro-government tabloid, published pictures of six opposition politicians on its front page with the headline: “They are threatening murder and to rape children.”

“The hatred and lies are constant,” said Dragan Djilas, an opposition leader who was one of the six pictured.

Mr. Vucic has since adopted a more conciliatory tone. On Wednesday, he said he was eager to start talks with his opponents and proposed holding early elections, an option opposition leaders reject because the playing field is so tilted against them.

Mr. Vucic also promised pay raises for teachers and health workers, and a cash payment for citizens under age 16, widely seen as a bribe to keep them and their parents off the streets.