The Student List, a leaderless movement of student-backed candidates, wants to end more than 14 years of rule by Aleksandar Vucic's right-wing populist Serbian Progressive Party (SNS).
The newly formed Student List is challenging the SNS and opinion polls suggest a close race between the parties.
The large crowd listened to speeches by 10 candidates at central Slavija Square on Thursday, news portal Danas.rs reported.
"We are tired of not everyone being equal before the law," Student List candidate and actor Tihomir Stanic told the rally.
Serbia deserved freedom, justice and a responsible government, he said.
Other speakers described the election as a choice "between Serbia and the mafia, between freedom and slavery, between prosperity and decline."
Critics accuse Vucic's government of allowing corruption to become entrenched. Vucic resigned as president in September in what was described as a tactical move to pave the way for him to become prime minister.
Critics also accuse him of bringing much of the judiciary under his control, using ruling party supporters to intimidate political opponents and restricting media freedom. Serbia has made little progress towards its goal of joining the European Union under Vucic's government.
The student list was formed by a new student movement that emerged from mass protests following the collapse of a railway station canopy in the northern city of Novi Sad in November 2024, which killed 16 people.
Critics blamed corruption and negligence under Vucic's government for the disaster.
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