The attacker used an automatic weapon to shoot randomly at people near the town of Mladenovac, some 50km south of the capital Belgrade on Thursday night, the Radio Television of Serbia (RTS) said.

Serbian police arrested the 21-year-old suspect identified only as U.B, following a manhunt dubbed Operation Whirlwind, the broadcaster confirmed on Friday morning.

The shooting comes less than 48 hours after a 13-year-old boy shot dead nine people at a school in Belgrade before turning himself in.

Serbian President Aleksandar Vucic subsequently proposed an array of tough measures to improve gun control and bolster security in schools in the Balkan country after two deadly mass shootings this week.

Serbian interior minister Bratislav Gasic described the latest shooting as a "terrorist act", Serbian news portal Telegraf reported, without providing further detail.

According to local media, after a late-night argument in a schoolyard near Mladenovac, the suspect returned with an assault rifle, opened fire and continued to shoot at people at random from a moving car.

No other details were immediately available, and police had not issued any statements.

Around 600 Serbian police officers, including elite Special Antiterrorist Unit (SAJ) and Gendarmerie, launched a manhunt to locate the suspect.

A Reuters witness saw heavily armed police establishing a checkpoint and searching incoming traffic, near the village of Dubona, not far from Mladenovac.

A helicopter, drones and multiple police patrols also searched for the suspect among the hills and forests around Dubona.

Eight children and a security guard were killed in Wednesday's shooting at Vladislav Ribnikar primary school in Belgrade.

Police said the 13-year-old boy had used two handguns he had taken from his father and had planned the attack for a month. They added that the boy drew sketches of classrooms and made lists of the children he planned to kill.

The shooting also left seven people hospitalised - six children and a teacher. One girl who was shot in the head remains in a life-threatening condition, and a boy is in serious condition with spinal injuries, doctors said on Thursday morning.

The country's first mass school shooting prompted the government to announce tougher rules on gun ownership.

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Serbia's president announced a moratorium on new gun licences except those for hunting, and a review of existing permits, with more surveillance of shooting ranges and of how people store their weapons.

He added that the government would hire 1,200 new police officers to improve security in the Balkan country's schools.

Police urged citizens to lock up their guns and keep them safe, away from children.

On Thursday, thousands of people gathered in central Belgrade to lay flowers and light candles to commemorate those killed in the shooting on Wednesday morning.

Gun culture is widespread in Serbia and elsewhere in the Balkans. The region is among the top in Europe in the number of guns per capita.