Serbia protests
Protestors with signs saying 'How much longer?' outside Serbia's Electoral Commission in Belgrade on Tuesday. AFP

Hundreds of people demonstrated against alleged voter fraud in the Serbian capital of Belgrade Tuesday, following calls by the main opposition camp to protest the weekend's election results that have been marred by reports of "irregularities".

The protest came a day after thousands rallied in front of Serbia's election commission following the vote that saw President Aleksandar Vucic claim a commanding victory in parliamentary and local elections.

The crowd on Tuesday was smaller than the previous protest, with demonstrators, including a large number of students, chanting anti-government slogans and hoisting signs targeting Vucic.

"I am here tonight because I am very angry because of the voter fraud, not only in Belgrade but in all of Serbia," Marko Radicevic, 29, told AFP.

Opposition leader Marinika Tepic indicated that protests would continue, saying more rallies would be needed to pressure the government.

"We caught them committing voter fraud, and we can't accept that," Tepic said.

Among the demands from the protestors, Tepic said opposition leaders were hoping to appear on a major state broadcaster, where coverage of the rallies and alleged fraud has not been shown.

Earlier in the day, the country's leading opposition coalition called on people from across the political spectrum to join the demonstration.

"We invite all opposition parties and movements, citizens and civil society to join us," said Miroslav Aleksic, a leader from the Serbia Against Violence (SPN) movement, at a press conference in Belgrade.

Criticism of the elections mounted after a team of international observers, including representatives from the Organization for Security and Co-operation in Europe (OSCE), denounced a string of "irregularities" including "vote buying" and "ballot box stuffing".

Germany later labelled the reported allegations "unacceptable" for a country hoping to join the European Union, while the United States called on Belgrade to address the "concerns" of the election monitors.

"Claims of irregularities reported both by OSCE and other election observation teams should be investigated, and violence directed at election authorities, journalists, accredited observers -- of which we have seen reports -- is unacceptable," US State Department spokesman Matthew Miller said.

The EU also joined the chorus of criticism, saying Serbia's "electoral process requires tangible improvement and further reform".

Even though Vucic was not personally on the ballot for the parliamentary and local elections over the weekend, the contest was largely seen as a referendum on his government.

Vucic's right-wing Serbian Progressive Party (SNS) earned roughly 46 percent of votes in the parliamentary elections, while the leading opposition coalition secured 23.5 percent of ballots, according to official results.

The SNS also said it won in municipal elections in the capital Belgrade, where the party faced their stiffest challenge from a loose coalition of opposition parties and candidates running under the SPN banner.

The SPN movement was formed in the wake of back-to-back mass shootings earlier this year, which spurred hundreds of thousands to take to the streets in rallies that morphed into anti-government protests over several months.

Opposition groups have cast doubts over the validity of the contest following accusations that the government allowed unregistered voters from neighbouring Bosnia to cast ballots illegally in the capital.