BELGRADE (Reuters) -Serbian police on Saturday evening clashed with anti-government protesters demanding snap elections and an end to the 12-year rule of President Aleksandar Vucic.

Police deployed scores of officers in riot gear around government buildings, parliament and nearby Pionirski Park, where throngs of Vucic's backers from across the country gathered in a counter-protest.

After the protest ended at around 10 p.m. (2000 GMT) some protesters who wanted to confront Vucic's backers threw flares at police, while the police used pepper spray to disperse them.

The protesters shouted: "Keep the shields down," calling on the police to stop intervening.

Months of protests across the country, including university shutdowns, have rattled Vucic, a populist, whose second term ends in 2027, when there are also parliamentary elections scheduled. 

Vucic's opponents accuse him and his allies of ties to organised crime, violence against rivals and curbing media freedoms, something they deny.

The student protest is set to last until 9 p.m. (1900 GMT) at Slavija Square and Nemanjina Avenue, where most of the government offices are located. 

The protesters, who want the government to heed their demands by the end of the protest, have pledged non-violence. 

Vucic has previously refused snap elections. His Progressive Party-led coalition holds 156 of 250 parliamentary seats.

On Saturday, Vucic said unspecified "foreign powers" were behind the protest. He said police should be restrained, but warned that violence will not be tolerated. 

“The country will be defended, and thugs will face justice,” he told reporters in Belgrade. 

Sladjana Lojanovic, 37, a farmer from the town of Sid in the north, said she came to support students. 

"The institutions have been usurped and ... there is a lot of corruption. Elections are the solution, but I don't think he (Vucic) will want to go peacefully," she told Reuters.

In the days ahead of the protest, police arrested about a dozen anti-government activists, charging them with undermining the constitution and terrorism. All denied the charges. 

Protests by students, opposition, teachers, workers and farmers began last December after 16 people died on November 1 in a Novi Sad railway station roof collapse. Protesters blame corruption for the disaster.

The Belgrade rally coincides with St. Vitus Day, venerated by most Serbs, which marks the 1389 Battle of Kosovo with Ottoman Turks.

(Reporting by Aleksandar Vasovic and Ivana Sekularac; Editing by Andrew Heavens, Jane Merriman and Daniel Wallis)