Chinese authorities have vowed to crack down on protesters in a central Sichuan province city after riot police clashed with hundreds of people rallying against a planned new metals plant.In unusually strong language, Shifang city police warned citizens on Tuesday that they would be "severely punished” if they sought to continue the "illegal” protests over environmental concerns.The violence erupted on Monday when, according to the official account, protesters in the city of around 430,000 attacked government offices with bricks and stones, smashed cars, and clashed directly with police and government employees.The city government has said only 13 people were injured."Anyone who has incited, planned or organised illegal gatherings, protest marches or demonstrations or those who have engaged in smashing and looting ... will be punished severely,” the police said. "Anyone using the Internet, mobile text messages and other methods to incite, plan or organise illegal gatherings, protest marches or demonstrations must immediately stop their illegal activities.”‘Bloodied demonstrators’Photos of the unrest that were posted online but which could not be verified appeared to show hundreds of police in riot gear clashing with protesters.Others showed hundreds of protesters marching through the streets carrying banners calling for the planned factory to be scrapped.