Several thousand people gathered outside the Central Election Commission building in Belgrade on Monday to protest against elections earlier this month that international observers say were rigged, Reuters reported.

The protesters went to the main police station, where, according to them, they were detained by law enforcement agencies.

Earlier in the day, police said 38 people were detained during and after an opposition protest against Sunday’s election results. The police reported that eight police officers were injured in the clashes.

On Sunday, protesters smashed windows and doors near the main entrance to City Hall, and police used pepper spray to disperse them around 10:00 p.m. (9:00 p.m. GMT).

Police Chief Ivica Ivkovich said that two of the eight policemen were seriously injured.

“We will continue to work to maintain peace and order, and we expect new arrests in connection with last night’s protests,” Ivkovic said at a press conference.

Russia, an ally of Serbia, said foreign-backed forces were trying to create trouble.

“There are tests and attempts by outside forces, including from abroad, to cause such riots in Belgrade,” Kremlin press secretary Dmytro Peskov said.

“We have no doubt that the leadership of the republic will ensure the rule of law.”

Opposition parties accused the police of using excessive force, and some social media showed footage of police officers beating men in the streets near City Hall.

The Serbian Prime Minister thanked the Russian special services

Prime Minister Ana Brnabych, who is resigning, thanked the Russian special services for providing information about the planned activities of the opposition.

“It (my statement) will not be popular in the West,” Brnabic of the ruling Serbian Progressive Party (SNS) told Serbian television.

Serbia, which aspires to join the European Union, has resisted pressure from Western countries to impose sanctions on Russia.

Moscow has been one of Serbia’s closest allies for decades, especially after 1999, when Russia opposed NATO airstrikes against the former Yugoslavia, which included Serbia and Montenegro.

Last Monday, an international monitoring mission said SNS was favored by media bias, undue influence of President Aleksandar Vučić and violations of voting rules, such as vote-buying. Serbian authorities deny any wrongdoing.

The ruling populist party, the Serbian Progressive Party (SNS), won 46.72 percent of the vote in early parliamentary elections on December 17, according to preliminary results from the state election commission.

Serbia Against Violence came second in the election with 23.56% of the vote, while the Socialist Party of Serbia came in third with 6.56%.