Dozens of armed riot police arrived at a factory in China this morning where workers were protesting over changes to their contracts.Staff at the Foxconn factory in Zhengzhou city, Henan province, clashed with the riot police on Tuesday evening and Wednesday morning. The facility is the company's main iPhone plant.One of the employees told local media that one of the reasons they protested was because the company changed their contracted secretly without their knowledge.The change affected their salaries, benefits, bonuses, and the contracted period.Managers sent in riot police who were seen clashing with the rebelling workforce.