At least eight people have been killed and 10 people have been wounded in a drive-by style shooting south of Belgrade on Thursday night, Serbian state TV reported.
The attacker used an automatic weapon to shoot randomly at people near the town of Mladenovac, 30 miles south of the capital, the Radio Television of Serbia (RTS) report said early on Friday.
Police were looking for the 21-year-old suspect, who fled after the attack, with the minister of police, Bratislav Gasic, treating the shooting as a terrorist attack.
RTS said the youngest person reported to have died was born in 2008.
They also said 600 special unit officers are involved in the search operation for the suspect and are using thermal imaging cameras in a hope to find the suspect.
The bloodshed sent shockwaves through the Balkan nation which is not used to such mass murders.
Dozens of Serbian students, many wearing black and carrying flowers, paid silent homage on Thursday to peers killed a day earlier.
The students filled the streets around the school in central Belgrade as they streamed in from all over the city.
Earlier, thousands had lined up to lay flowers, light candles and leave toys to commemorate the eight children and a school guard who were killed on Wednesday morning.
The tragedy sparked a debate about the general state of the nation following decades of crises and conflicts which has created a state of permanent insecurity and instability, along with deep political divisions.
Authorities on Thursday moved to boost gun control, as police urged citizens to lock up their guns and keep them safe, away from children.