Around 2,000 people showed up towards the end of a warm and sunny day to attend "La Boum" (the party) in the Bois de la Cambre park, local police said, even though the organizers had revealed days earlier that their invitation on social media was a hoax. Local media put the number of people at about 5,000.
Hundreds of police, many in riot gear and some on horseback, broke up a knot of would-be revelers beside a lake in the park with back-up from water cannon trucks and tear gas.
Many in the crowd pumped their fists and chanted "freedom" and some threw bottles and stones at the water cannon trucks.
Reuters reporters at the scene said at least two civilians were injured in clashes, including one woman who was knocked over by a police horse. A police spokeswoman said three policemen were injured and four people were arrested.