The strike at Foxconn's Zhengzhou factory began at 1 p.m. local time and involved 3,000 to 4,000 workers, according to New York-based China Labor Watch, which said it received its information from workers at the plant.
The workers are upset about stricter quality-control requirements introduced for the new Apple smartphone, the labor group said. They are also upset at being made to work through a Chinese national holiday this week, it said.
"According to workers, multiple iPhone 5 production lines from various factory buildings were in a state of paralysis for the entire day," China Labor Watch said. It also reported that quality-control inspectors were attacked.
Apple did not immediately respond to a request for comment and Foxconn, based in Taiwan, could not be reached for comment. It was not possible to confirm independently the number of workers striking.
It wouldn't be the first disruption at a Foxconn plant in China. Last month, at a factory in Taiyuan after what workers described as aggressive behavior from security guards.
This latest incident followed tighter rules to prevent tiny indentations on the phones and scratches to the phones' frames and back covers. The new iPhone 5 is said to be more susceptible to such markings.
A fight between workers and quality-control inspectors resulted in some injuries and people being taken to the hospital, China Labor Watch said.
"They have such high expectations for these products, even if you raise the demands a little bit it makes a huge difference to the pressure on the workers," Li Qiang, China Labor Watch's executive director, said in an interview.
He last heard from workers at the plant late Friday night and the strike had not been resolved, he said.