Police have begun destroying vegetable gardens planted by Zimbabwe's urban poor, extending a demolition campaign that initially targeted shacks and street vendor kiosks. Senior assistant police commissioner Edmore Veterai said urban farming on vacant plots of land was causing "massive environmental damage," state radio reported Tuesday. The crackdown _ at a time of food shortages in Zimbabwe _ is the latest escalation in the government's monthlong Operation Murambatsvina, or Drive Out Trash, which has seen police torch the shacks of poor city dwellers, arrest street vendors and demolish their kiosks. President Robert Mugabe was quoted Tuesday as saying concern about the campaign was misplaced and agreeing to allow in a U.N. observer. Mugabe defends the campaign as a cleanup drive. The political opposition, which has its base among the urban poor, says the campaign is meant to punish its supporters. The United Nations estimates the campaign has left at least 1.5 million people homeless in the winter cold. Police say more than 30,000 have also been arrested, most of them street vendors the government accuses of sabotaging the failing economy by selling black market goods.