BELGRADE – A deafening sound of whistles and vuvuzelas echoed throughout downtown Belgrade on Saturday as Serbia’s capital geared up for a major rally against populist President Aleksandar Vucic and his government.

Huge crowds have been converging on downtown Belgrade for the gathering seen as a culmination of months of almost daily anti-corruption protests that have posed the biggest challenge to Vucic’s 13-year firm grip on power in Serbia.

Reflecting mounting tensions, police said they arrested a man who rammed his car into protesters in a Belgrade suburb, injuring three people.

Ahead of the demonstration, Vucic repeatedly warned of alleged plans for unrest while threatening arrests and harsh sentences for any incidents.

In an apparent effort to prevent people from attending the rally, Belgrade city transport was canceled Saturday while huge columns of cars jammed the roads leading into the capital. The transport company said the cancellation was made “for security reasons.”

On Friday evening, tens of thousands of people staged a joyous welcome for the students who have been marching or cycling for days from across Serbia toward Belgrade for the main rally on Saturday afternoon. From early morning, people started assembling in various parts of the city, preparing to march toward the center.

Fueling fears of clashes, Vucic’s supporters have been camping in central Belgrade in front of his headquarters. The crowd included ex-members of a dreaded paramilitary unit involved in the assassination in 2003 of Serbia’s first democratic Prime Minister Zoran Djindjic, as well as soccer hooligans who are known for causing violence.

Private N1 television on Saturday broadcast a footage of dozens of young men with baseball caps going into the pro-Vucic camp.

Interior Minister Ivica Dacic told state RTS broadcaster that 13 people have been detained overnight but that no major incidents were reported on Friday. He said police detained six opposition activists for allegedly plotting to stage a coup and stir unrest on Saturday.

Protesting 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 on Nov. 1.

Many in Serbia blamed the crash on rampant government corruption, negligence and disrespect of construction safety regulations.

Vucic has been claiming that Western intelligence services were behind almost daily student-led protests with an aim to oust him from power.

Students have struck a chord 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.