The reported strike comes at a crucial time for the U.S. corporation, weeks after kicking off its largest-ever global rollout of the smartphone. Apple is already struggling with supply constraints, analysts say.
Citing workers, the labor group said 3,000 to 4,000 workers began their strike at Foxconn's Zhengzhou complex in the afternoon, incensed by over-exacting quality controls as well as demands they work through the week-long "Golden Week" holidays, which began Monday.
The strike could not be immediately confirmed. Apple declined to comment and Foxconn was not immediately available for comment.
Tensions have boiled over repeatedly in factories operated by Foxconn, the Taiwanese contract manufacturing giant that employs more than a million and makes most of the world's iPads and iPhones. Last month, thousands rioted at its Taiyuan facility in northern China, disrupting production for about 24 hours and underscoring the potential for labor unrest.
"In addition to demanding that workers work during the holiday, Foxconn raised overly strict demands on product quality without providing worker training for the corresponding skills," the Watch said in a statement on its website (http://www.chinalaborwatch.org/news/new-433.html).
"Additionally, quality control inspectors fell into conflicts with workers and were beat up multiple times by workers. Factory management turned a deaf ear to complaints about these conflicts and took no corrective measures."
The group did not say in its release when work might resume.
Foxconn Technology Group of Taiwan, the trading name of Hon Hai Precision Industry Co <2317.TW>, is the world's largest contract maker of electronics for global brands such as Hewlett Packard Co , Nokia and Dell Inc .
Apple and Foxconn have come under fire for poor working conditions and wages at plants across China. In response, they have organized an audit of factory conditions, raised wages, improved safety and reduced overtime, among other measures.
(Reporting By Edwin Chan; Editing by Richard Chang)