BRUSSELS—Police used teargas and water cannons on Thursday to disperse thousands of young people gathered in a Brussels park for a party in defiance of Belgium’s COVID-19 lockdown, an event that began as an April Fool’s joke on Facebook. Around 2,000 people showed up toward 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 teargas. Many in the crowd pumped their fists and chanted “freedom” and some threw bottles and stones at the water cannon trucks. “We …