The Government is targeting hot food stands owned by migrant hawkers in a bid to curb urban pollution. Authorities yesterday imposed strict rules in the capital to restrict trade by migrants. 'It takes time and effort for these stands to vanish,' said the Beijing Administration for Industry and Commerce. Officials imposed an immediate ban on barbecue stalls along all main avenues and threatened to close a third of Beijing's open-air markets as part of a 'city beautification drive'. 'Smoke generated by outdoor cooking threatens residents and passers-by,' a city environmental agency said. Beijing has nearly four million migrants in a population of 11 million, representing the largest transient population of any city in China. Authorities have repeatedly blamed migrants for street crime, traffic congestion and 'social instability' in Beijing, Shanghai and other big cities. 'Vendors are also accused of adding to pollution,' said China Daily, which described the crackdown on transients in the capital as a 'clean-up effort'. The edict on migrants follows a year-long campaign for restrictions on them as an urban hazard by the Beijing Environmental Protection Bureau. The bureau claims smoke from migrants' roadside stalls exceeds federal air pollution standards and contributes to 'the formation of acid rain'.