The Serbia Against Violence group - a coalition of opposition political parties - has accused President Aleksandar Vučić's populist government of orchestrating a fraud in the 17 December parliamentary and local elections, particularly in the capital Belgrade.
Vučić has denied the accusations levelled against him.
The state election commission has declared Vučić's Serbian Progressive Party the election winner and rejected the opposition's complaints. The Constitutional Court is yet to rule on the dispute.
Opposition politician Radomir Lazovic said "this is a battle of all of us, everyone loses with this [vote rigging]."
Some 2,000 people also marched toward the headquarters of Serbia's public broadcaster RTS, which they accuse of pro-government bias.
International election monitors reported instances of vote buying and ballot box stuffing, and said the Serbian election generally was held in “unjust conditions.”
Opposition parties and local independent election observers have claimed voters were bused into Belgrade to cast ballots for the ruling party after being registered at bogus addresses.
Thousands of people rallied for days in late December accusing Vučić of stealing the ballot. Police used pepper spray and detained dozens of people after protesters tried to enter the Belgrade City Hall at one of the demonstrations.
Though Vučić says he wants to lead Serbia into European Union membership - for which it has been a contender since 2012 - critics accuse the populist leader of imposing an autocracy by taking control over mainstream media and all state institutions.
The EU, for its part, has said Serbia must boost rule of law and democracy freedoms in order to become a member of the 27-nation bloc.