The Associated Press reported that the small fires at the embassy were quickly extinguished.

Serbian television reported that the Croatian Embassy had also been attacked, and the state news agency said that the Bosnian and Turkish Embassies were also targets. The police said at least 140 people had been injured in the incidents, 32 of them police officers. Security sources estimated that 150,000 people joined the protests.

Image Protesters in Belgrade set fire to an American flag on Thursday. Small fires were also set inside the United States Embassy. Credit... Ivan Milutinovic/Reuters

Groups also broke into a McDonald’s in central Belgrade and destroyed its interior. Witnesses said vandals were attacking foreign-owned shops, including a Nike store, and were seen carrying off shoes and other goods as the Serbian police looked on.

The United States Embassy had been closed since Sunday after it was stoned.

R. Nicholas Burns, the under secretary of state for political affairs, telephoned Serbian officials to formally complain about the breaching of the embassy, said a State Department spokesman, Sean McCormack. Mr. McCormack told reporters on Thursday that “we would hold the Serbian government personally responsible for the safety and well-being of our embassy employees.”

He added that the security that had been provided was completely inadequate.

The United Nations Security Council issued a unanimous statement of the 15 members saying they “condemn in the strongest terms the mob attacks against embassies in Belgrade which have resulted in damage to embassy premises and have endangered diplomatic personnel.” The action was taken at the urging of Zalmay Khalilzad, the American ambassador.

The violence fueled fears in Washington and Brussels that Serbia was turning to the virulent nationalism of the past. But Serbian analysts predicted the country would ultimately embrace the West as it came to terms with losing its medieval heartland.