Videos online showed thousands of people in masks facing rows of police in white protective suits with plastic riot shields. Police kicked and hit a protester with clubs after he grabbed a metal pole that had been used to strike him. People who made the footage said it was filmed at the site.
The protest in Zhengzhou lasted through Wednesday morning as thousands of workers gathered outside dormitories and confronted factory security workers, according to Li.
Other videos showed protesters spraying fire extinguishers toward police.
Foxconn, the biggest contract assembler of smartphones and other electronics, is struggling to fill orders for the iPhone 14 after thousands of employees walked away from the factory in the central city of Zhengzhou last month after complaints about unsafe working conditions.
Protests started on Tuesday after employees who had travelled long distances to take jobs at the factory complained that the company changed the terms of their pay, according to an employee, Li Sanshan.
Li said he quit a catering job when he saw an advertisement promising 25,000 yuan ($3,500) for two months of work. That would be significantly above the average pay for this type of work in the area.
After employees arrived, the company said they had to work two additional months at lower pay to receive the 25,000 yuan, according to Li.
A man who identified himself as the Communist party secretary in charge of community services was shown in a video posted on the Sina Weibo social media platform urging protesters to withdraw. He assured them their demands would be met.
The protests come amid severe frustration over Covid restrictions in areas across China that have closed shops and offices and confined millions of people to their homes.
China reported its highest number of new Covid cases on Thursday, with 31,444 new infections.
On Wednesday, the government ordered an effective lockdown of several districts of Zhengzhou, with residents of the city centre not allowed to leave unless they have a negative Covid test and permission from authorities.
The company did not immediately respond to a request for comment on the latest protests.
Reuters and Associated Press contributed to this report
