In a statement late on Thursday, the police said they had rounded up migrants in the municipalities of Subotica, Sombor and Kikinda near the Hungarian border in the north and near the town of Pirot in the southeast, near the border with Bulgaria.
Last week three migrants died in a shootout near Serbia's border with Hungary, a route increasingly used by people smugglers for entering the European Union.
The police said they had searched more than 5,000 cars and 32 homes and had uncovered passports and other personal documents. So far theh have rounded up hundreds of migrants and arrested dozens.
"In the coming days, police will step up measures and engage forces ... to suppress illegal migration," the statement said.
It did not say where the migrants had come from but most of those using the Balkan route into the EU come from the Middle East, Afghanistan, Pakistan and north Africa. The route runs via Turkey, Bulgaria, North Macedonia and Serbia.
Many migrants are crossing borders with the help of elaborate networks of smugglers who are sometimes armed, and shootouts between criminal groups are frequent.
Serbia conducts joint border patrols with EU members Hungary and Austria. Belgrade has pledged to align its visa policies with those of the EU to help stem the flow of illegal migrants westward.
(Reporting by Aleksandar Vasovic; Editing by Gareth Jones)
