A team of international observers on Monday criticised Serbian elections over a string of “irregularities” that cemented President Aleksandar Vucic and his party's continued dominance over the country.
Allegations of “vote -buying” and “ballot box stuffing” were made after the opposition accused the ruling party of voter fraud.
The concerns voiced by the monitors, which included representatives from the Organisation for Security and Co-operation in Europe, came hours after Mr Vucic said his party had secured a commanding victory during parliamentary and local elections.
“Election day was smooth but marred by isolated instances of violence, procedural irregularities and frequent allegations of organising and busing of voters to support the ruling party in local elections,” the International Election Observation Mission said in a statement.
“Further instances of serious irregularities, including vote-buying and ballot box stuffing were observed.”
Sunday's vote came weeks after Mr Vucic called for snap elections in November, the latest example of how governments under his rule rarely serve out their term – a move critics say is designed to keep the opposition off balance.
Mr Vucic and his ruling right wing Serbian Progressive Party (SNS) looked poised to tighten their grip on power after claiming to have captured 127 out of the 250 seats in parliament late on Sunday.
The SNS won an outright majority in parliament with 47 per cent of the vote, while Serbia Against Violence, a liberal challenger seeking to dent Mr Vucic’s grip on power, took 23 per cent, the Electoral Commission said after counting almost all ballots. The Socialist Party, a Vucic ally, won 6.6 per cent.
“It’s an absolute victory,” Mr Vucic told supporters in Belgrade after polls closed. He swore to unite the country and stick with Serbia’s EU path while refusing to recognise neighbouring Kosovo as independent.
However, 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 Belgrade during the election.
The SNS also said it had secured a win in municipal elections in the capital, where the party faced their stiffest challenge from a loose coalition of opposition parties and candidates running under the Serbia Against Violence banner.
That movement was formed following back-to-back mass shootings earlier this year that spurred hundreds of thousands to take to the streets in rallies that morphed into anti-government protests over several months.
Those involved in the movement vowed to take to the streets again, calling for supporters to rally on Monday night against what the group called “electoral theft” during local elections in the capital.
The government has dismissed the claims, with Prime Minister Ana Brnabic saying the allegations were designed to spread chaos.
Even though Mr Vucic was not personally on the ballot during the parliamentary and local elections at the weekend, the contest was largely seen as a referendum on his government.
“My job was to do everything in my power to secure an absolute majority in the parliament,” he said as he celebrated what he said was the SNS's victory late on Sunday.
To his supporters, Mr Vucic's decade in power has brought stability and billions in investments to the once chaotic country ravaged by a string of wars and bouts of hyperinflation in the 1990s.
But his opponents have long accused the President and the SNS of overseeing a government defined by autocracy and corruption.
Hungarian Prime Minister Viktor Orban praised Mr Vucic, calling the win an “overwhelming election victory”.
Mr Vucic has been particularly deft at balancing ties between East and West, vowing to keep Serbia on a course for EU membership – while also remaining friendly with Russia and courting China and Washington.
On Monday, Moscow congratulated Mr Vucic and the SNS on their victory, with Kremlin spokesman Dmitry Peskov saying he hoped the win would lead to the “further strengthening of friendship” between the countries. He referred to Serbia as a “brotherly” country.
Serbia and Russia have historically close ties and Belgrade did not join international sanctions against Moscow for its Ukraine offensive.
A considerable part of Serbia's population is supportive of Moscow's large-scale military campaign in Ukraine, sharing Russia's dislike of the US-led Nato military alliance that backs Kyiv.
In addition, Serbia is almost entirely dependent on Russian gas.
But Belgrade has also become a hub for Russians critical of the offensive fleeing Moscow, with flights still open between the two countries, unlike from Russia to much of Europe.
But Serbia has condemned Russia's aggression at the UN and its support has caused controversy.
In November, Serbia's intelligence chief Aleksandar Vulin – who visited Russia during its offensive – resigned, claiming international pressure on Belgrade.
