Former Slovak Prime Minister Robert Fico was detained on Thursday on suspicion of organising an anti-government rally that had been banned due to lockdown rules.
Fico's leftist Smer-Social Democracy party and police confirmed the detention that took place before the planned demonstration in the Slovak capital Thursday evening.
Police said they would not give more details until Friday. It was not clear whether Fico would face charges.
Any public gatherings of more than six people and protests have been banned after the government imposed strict restrictions to tackle a record surge of coronavirus infections in late November.
Fico announced a protest on Thursday evening in the form of a car ride through the city.
However, when he arrived at the Tyrsovo embankment, he was detained by police and taken to the police station.
"This is what democracy on the air looks like, ladies and gentlemen. So once again, we wanted to make a peaceful passage through Bratislava and look at this situation," he said to journalists.
Fico, a populist leader, was forced to resign in 2018 after the murder of an investigative journalist exposed corruption and sparked an anti-government wave.
Jan Kuciak had revealed murky links between businessmen, politicians and other senior officials.
Marian Kocner, a multi-millionaire entrepreneur, was accused of ordering the murder. He was acquitted, but the Supreme Court overturned the ruling earlier this year and called for a retrial.
Fico has been a vocal opponent of the government's response to the pandemic and opposed vaccination.
The pandemic became a major challenge for the current coalition government that took power after beating Fico in the parliamentary elections early last year.
Earlier this year, Slovakia's top court ruled that a nationwide referendum cannot be held on whether to call an early parliamentary election.
President Zuzana Caputova had asked the Constitutional Court to rule on the matter after over 585,000 Slovak citizens signed petitions calling for the snap vote, which the political opposition proposed over the government's handling of the coronavirus pandemic.
Slovakia has been one of the hardest-hit European Union countries, with over 794,000 infected and 15,730 deaths.
Only 46.8% of the country's 5.4 million population is fully vaccinated, well below the EU/EEA average of 67.3%, according to the European Centre for Disease Prevention and Control (ECDC).
It is the third-lowest rate in the bloc after Bulgaria and Romania.