The crowds protested in the capital Belgrade over the government's handling of the crisis, with infections now spiking after Serbia shed its initial lockdown measures two months ago.
A group of opposition supporters stormed the Serbian parliament building in Belgrade on Tuesday night in a protest against a lockdown planned for the capital this weekend to halt the spread of the coronavirus.
Stricter measures for virus
Serbian President Aleksandar Vucic said on Tuesday evening that stricter measures including the lockdown of Belgrade over the weekend would be introduced because of the rising number of coronavirus infections.
Opponents blame the increase on the government and say people should not have to pay the price for another lockdown.
Skirmish with police
After Vucic's statement, several thousand people began gathering in front of the parliament in Belgrade's central square.
Around 1800 GMT., a small group of protesters pushed past a police cordon, broke through a door and entered the parliament building. But police later pushed them back.
The demonstrators, who also lit flares and were seen throwing stones at police on local TV, chanted for President Aleksandar Vucic to "Resign!", police unleashed tear gas canisters to disperse the crowd.
"People gathered spontaneously. Discontent can be felt in the air," Radomir Lazovic of the Do Not Let Belgrade Drown opposition group told N1 television.
Serbian police and protesters, angered by the announced lockdown over the weekend, have clashed outside of the country's parliament building in Belgrade.— Balkan Insight (@BalkanInsight) July 7, 2020
Tear gas has been used to disperse the crowd. pic.twitter.com/guDsnGWt6Y
Delayed response to crisis
Vucic announced the return of a round-the-clock weekend curfew earlier in the evening after the Balkan state recorded its deadliest day yet, with 13 fatalities.
Critics accuse the president of racing to lift restrictions ahead of a national election on June 21, which his party won resoundingly.
In the weeks before the poll, there were virtually no anti-virus restrictions and sporting events were held with thousands of spectators.
Serbia, a country of 7 million people, has reported 16,168 coronavirus infections and 330 deaths. But the numbers are spiking and 299 cases and 13 deaths were reported just on Tuesday.
Epidemiologists and doctors warned that hospitals were running at full capacity and that medical workers were tired.
"Nobody can endure these numbers. We don't want to kill our doctors," said the president.
In early March, Serbia introduced a lockdown to halt the spread of the coronavirus.
But in late May, the Balkan country was among the first to open up and set elections for June 21.
Top party officials, including the president's adviser, were infected after celebrating their election victory in a small room while not wearing masks.
Opposition parties, many of which boycotted the election, criticise Vucic for using the lockdowns to strengthen what they call his autocratic rule.
Last week, the government imposed some new restrictions in several hotspot cities and towns, including Belgrade, where wearing masks in indoor spaces was made obligatory.
Several hospitals in the capital were also re-designated virus treatment hubs, and a field hospital has been reopened in a sports arena.
According to local media, the situation was particularly serious in the southwestern city of Novi Pazar, where hospitals have reported an overflow of patients and a lack of equipment.
Countries across the Balkans are now seeing an uptick in cases after initial success in reining in their Covid-19 outbreaks.
On Monday, neighbouring Kosovo reinstated nightly curfews in the capital Pristina and three other towns.