Between 275,000 and 325,000 people participated in the massive demonstration, according to an organization that calculates the size of crowds.
"Possibly, the figure may be even higher," the organization says in a statement.
The figures are significantly higher than the authorities' estimate of 107,000.
The protests originated from the roof collapse at a train station in Novi Sad in November, where 15 people died, and are directed against corruption and the country's government with President Aleksandar Vucic at the helm.
Crowds of flag-waving demonstrators gathered around Belgrade on Saturday to converge in front of the parliament. Some carried banners with the text "He is finished", referring to Vucic.
The massive demonstration – which at one point stretched over two kilometers – followed four months of student-led protests.
Highest Alert
The city had introduced highest alert and all public transportation was suspended. Hundreds of police officers were deployed to guard government buildings.
Vucic had previously threatened with arrests and harsh penalties for all disturbances in connection with the demonstrations.
Counter-demonstrators, including ultranationalists and alleged football hooligans, had gathered at the parliament.
The protests continued mostly in calm forms. However, the police reported in the evening that there had been "incidents and clashes between some of the participants".
At 19.20 local time, a leading student group urged the demonstrators to leave the area around the parliament. The situation was described as tense after stones and bottles were thrown.
Criticism from the EU
Already on Friday, demonstrators arrived in Belgrade and 13 people were arrested during the night, according to the Ministry of the Interior. Six activists are accused of planning a coup and attempting to incite unrest. A man who drove into a crowd with a car and injured three demonstrators has also been arrested.
The EU has long criticized Serbia, which has been negotiating EU membership since 2014, for its lack of democratic reforms.
