Thousands converging on Serbia's capital for a major rally that tests Vucic's populist government

BELGRADE, Serbia (AP) — Tens of thousands of people swarmed the streets in Serbia's capital Belgrade on Friday, staging a joyful prelude to a major anti-government rally on the weekend and dealing a blow to populist President Aleksandar Vucic.

The citizens of Belgrade came out to welcome thousands of university students who converged on Belgrade from across the country for the rally planned for Saturday that is expected to draw even more people.

Saturday’s protest is seen as a culmination of months of anti-graft demonstrations in the Balkan country that have posed the biggest challenge so far for Vucic’s decade-long firm grip on power in Serbia.

Flares, fireworks and flag-waving crowds filled the downtown streets in stark contrast to weeks of fear-mongering spread by Vucic's populists with an aim to dissuade people from attending the rally Saturday.

The autocratic leader has repeatedly warned that violence is planned at the rally and threatened arrests over any incidents. Vucic's supporters have been camping in the city center, fueling fears of clashes with the protesters.

At a news conference on Friday evening, a defiant Vucic again said that authorities have received multiple reports of alleged planned unrest and described Saturday's rally as “illegal.”

Vucic reiterated claims that Western intelligence services were behind the protests with an aim to oust him from power.

“I do not accept blackmail, I will not agree to pressure, I am the president of Serbia and I won't allow the street to set the rules,” Vucic said.

Students have led the nationwide anti-graft movement, which started after a concrete canopy collapsed at a train station and killed 15 people in Serbia’s north more than four months ago.

Earlier, police detained six opposition activists on suspicion of “preparing actions against the constitutional order and security” a day after an alleged recording of their meeting was broadcast on pro-government television stations on Thursday.

“The Serbian state will do everything to secure peace,” Vucic said. “Those disrupting peace will be arrested and severely punished.”

The protesters didn't seem to care. Marko Vukovic, a student from Belgrade said he came to “support my friends, colleagues and all the citizens who think for themselves, to fight for a more just Serbia.”

Many people cried, laughed and screamed with joy. Protesting students have struck a cord among the citizens who are disillusioned with politicians and have lost faith in the state institutions.

Previous student-led rallies in other Serbian cities have been peaceful while drawing huge crowds.

In Belgrade, tensions soared after Vucic's supporters, including former paramilitary fighters, set up a camp in a park outside the presidency building. Tractors were parked around the camp on Friday.

Authorities said that the parliament building across the street would be locked for the next three days because of security reasons. Railway traffic was suspended, along with many bus links to Belgrade.

Responding to tensions, the European Union's mission in Serbia said on Friday that “freedom of assembly is a fundamental right” and the safety of participants and institutions must be ensured.

“Violence must be avoided,” the EU said.

Serbia is formally seeking EU membership, but the governing populists have been accused of stifling democratic freedoms while strengthening relations with Russia and China.

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Associated Press writer Dusan Stojanovic contributed to this report.