Hundreds of police officers, some in riot gear, deployed after staff at the factory in the manufacturing hub of Shenzhen walked out on Tuesday and blocked a highway to protest long working hours, China Labour Watch said.
Staff at the plant commonly worked 100 to 120 hours of overtime a month and said they also suffered a high rate of workplace injuries, mass layoffs of older workers and frequent verbal abuse by managers, the US-based group said.
The plant, which employs 3 000 people in the Pearl River Delta - home to millions of Chinese migrant workers - is owned by Taiwan's Jingyuan Computer Group, which supplies components to Apple and IBM.
The strike ended after the company promised to reduce the amount of overtime, the rights organisation said in a statement. No one at the factory could immediately be reached for comment.
It was the latest in a recent spate of worker action in China, where strikes have historically been relatively rare as the country lacks truly independent trade unions.
More than 400 female workers at a bra factory in Shenzhen, which borders Hong Kong, cut off the power and downed tools this week after a manager told one to "jump off a roof and go to hell".
And last week, more than 7 000 workers at a factory in nearby Dongguan making New Balance, Adidas and Nike shoes went on strike, clashing with police in a protest over layoffs and wage cuts.