Police on horseback and using water cannon charged a crowd of up to 2,000 people gathered in a Brussels park on Friday for a fake concert announced on social media as an April Fool's Day prank.
AFP journalists at the scene saw at projectiles thrown at police in riot gear in the Bois de Cambre park on the southern side of the Belgian capital.
Police said that three officers were wounded, one of whom was taken to hospital, and four people were arrested.
The police, wearing protective helmets and advancing in a line, moved in to enforce strict Covid-19 social-distancing rules that prohibit gatherings of more than four people outdoors.
Brussels law enforcement authorities on Wednesday had issued a warning that the announcement on social media of a "party" was illegal and that its organisers could be prosecuted.
Belgium on Saturday imposed tighter restrictions aimed at curbing surging Covid infection numbers.
They include closing schools, keeping borders closed, limiting access to non-essential shops and lowering the number of people able to meet outdoors to four.
One participant, Selim Jebira, told AFP that "we were tear-gassed for no reason at all".
Brussels mayor, Philippe Close, tweeted that, while he could understand people wanting to go outside in the springtime weather, "we can't tolerate such gatherings".
He thanked the police "for the difficult job, and for people who have respected the rules for more than a year," since the start of the pandemic.