The iPhone 5, the latest in the line of the smartphones, debuted in September.
China Labor Watch said the Foxconn workers were required to work during the eight-day holiday from Sept. 30 to Oct. 7 to meet the production demand of iPhone 5.
Foxconn said its employees in China who worked during the holiday did so voluntarily and were being paid three times their normal pay, in accordance with Chinese labor law.
China Labor Watch said workers also beat quality control inspectors, who carried out their own work stoppage after management ignored their complaints.
According to China Labor Watch, Apple and Foxconn had imposed stricter quality standards regarding indentations and scratches on the frames and back covers of the iPhones but did not provide workers with proper training to meet the new demands.
Apple could not be reached immediately for comment.
In late September, a brawl involving 2,000 workers broke out at Foxconn's factory in the northern Chinese city of Taiyuan, highlighting chronic labor tensions in a country that prohibits independent unions.
Labor activists have said the rollout of the iPhone 5 has led to longer working hours and more pressure on workers.