BEIJING — Several hundred people protesting the planned expansion of a petrochemical plant in the Chinese port city of Ningbo swarmed the city center for a third day Sunday, refusing to back down even after the government promised to cancel the project.

As demonstrators outside the government offices refused to disperse, riot police with helmets and shields came out of the compound and pushed the crowd back. Police dragged six men and one woman into the compound, beating and kicking at least three of them, the Associated Press reported. Police also smashed placards and took away flags.