President Aleksandar Vucic called for the snap vote last month, barely two years since the last parliament was elected. His ruling party was expected to secure a new four-year term, but could struggle in local elections.
Serbia's governing populists claimed a sweeping victory at snap parliamentary elections on Sunday.
With 80% of the ballots counted, Ipsos and Belgrade-based pollster Cesid both projected the Serbian Progressive Party (SNS) to win 46% of the vote while the largest opposition group, Serbia Against Violence (SPN), was projected to win 23%.
"We will have an absolute majority in parliament with 127 seats," President Aleksandar Vucic told reporters.
Vucic's name was not on the ballot, as it was not a presidential election, but the vote was widely seen as a referendum on his leadership after a turbulent year for the country, which saw two back-to-back mass shootings that claimed the lives of 18 people.
The shootings rattled Vucic's support, yet his right-wing populist SNS maintained a commanding lead over the opposition.
Over the past year tensions with neighboring Kosovo, which declared independence in 2008, have also escalated.
Observers flag voting irregularities
Some electoral observers and independent media reported irregularities on voting day.
One report claimed that ethnic Serbs from Bosnia and Herzegovina gathered to vote in a sports hall in Belgrade that was not an official polling station.
Observers from the Center for Research, Transparency and Accountability expressed "the highest concern" over allegations that illegal voters were bussed in to Belgrade from other countries.
"The concentration of buses, minivans and cars was observed on several spots in Belgrade, transferring voters to polling stations across the city to vote," the civil society watchdog said.
It also reported cases of voters being given money to vote for the SNS.
Authorities have rejected claims of any wrongdoing and Acting Prime Minister Ana Brnabic dismissed the reports as "lies that are intended to spread panic."
Ruling party challenged in local elections
The parliamentary vote coincides with local elections taking place in most municipalities, the capital Belgrade and the northern province of Vojvodina.
The Belgrade vote is shaping up to be a crucial battleground between the SNS and centrist SPN coalition.
The movement was formed in the wake of the mass shootings, which pushed hundreds of thousands of anti-government protesters to the streets for months.
Vucic has dismissed the rallies as a foreign plot, often warning that Serbia would be left directionless without him.
"It's not about me leaving power, but about them destroying everything," he told supporters at a recent rally.
"It would take us 20 years to fix everything [...] That's why we'll beat them more convincingly than ever."