BRUSSELS—Police fired water cannon and tear gas in a Brussels park on Saturday to break up an anti-lockdown party of several hundred people designed to defy coronavirus social distancing rules. The crowd of mostly young people responded to a post on Facebook announcing the unauthorised party. It took place a month after police cleared 2,000 people who gathered in the same Bois de la Cambre park for la Boum (the party), an event that had begun as an April Fool’s joke. The follow-up Boum 2 event on May 1, a traditional day for demonstrations, was held a week before the Belgian government allows cafe and bar terraces to open and lets groups of more than four people meet outside in relaxation of COVID-19 rules. Prime Minister Alexander De Croo urged Belgians on Friday to stay united and not “fall into this trap”. Facebook also took down the Boum 2 post …