Taiwan's Foxconn Technology Group, which assembles Apple's iPhones and makes components for top global electronics companies, closed a plant in China on Monday after about 2,000 workers were involved in a brawl at a company dormitory.
It was not clear how long the shutdown would last at the plant, which employs about 79,000 people in the northern Chinese city of Taiyuan, while police and company officials investigate the cause of the disturbance.
Foxconn said the trouble started with a personal row that blew up into a brawl. But some people posting messages on a Twitter-like site said factory guards had beaten workers and that sparked the melee.
The plant is closed today for investigation, Foxconn spokesman Louis Woo told Reuters. An employee contacted by telephone said the closure could last two or three days.
Pictures from just outside the plant and provided to Reuters showed broken windows at a building by an entrance gate and a line of olive-coloured paramilitary police trucks parked inside the factory grounds.
The unrest is the latest in a string of incidents at plants run by Foxconn, the trading name of Hon Hai Precision Industry Co and the world's largest contract maker of electronic goods. Hon Hai's Taipei-listed shares fell 1 percent on Monday in a broader market that rose 0.2 percent.
Drawing attention as a supplier and assembler for Apple products, the company has faced accusations of poor conditions and mistreatment of workers at its operations in China, where it employs about 1 million workers.
The company has been spending heavily in recent months to improve working conditions and to raise wages.
Foxconn said in a statement the incident escalated from what it called a personal dispute between several employees at around 11 p.m. on Sunday in a privately managed dormitory, and was brought under control by police at around 3 a.m.
The cause of this dispute is under investigation by local authorities and we are working closely with them in this process, but it appears not to have been work-related, Foxconn said. Hon Hai said about 2,000 workers were involved.
Comments posted online, however, suggested security guards may have been to blame.
In a posting on the Chinese Twitter-like microblog site Sina Weibo, user Jo-Liang said that four or five security guards beat a worker almost to death.
Another user, Fan de Sa Hai, quoted a friend from Taiyuan as saying guards beat up two workers from Henan province and in response, other workers set bed quilts on fire and tossed them out of dormitory windows.
The accounts could not be independently confirmed.
The state-run Xinhua news agency quoted a senior official with the Taiyuan city government as saying investigators initially determined the fight broke out as workers from Shandong Province clashed with workers from Henan.
The agency reported earlier that about 5,000 police were sent to end the violence, according to Taiyuan City's public security bureau.
Foxconn cited police as saying 40 people were taken to hospital and a number were arrested, while Xinhua added that three people were in serious condition.
Calls to the Taiyuan police were not immediately answered, while an official at the plant declined to comment when reached by telephone.
Clearly there is deep-seated frustration and anger among the employees and no outlet, apart from violence, for that frustration to be released, Geoff Crothall, communication director at China Labour Bulletin, a labour rights group in Hong Kong, said in a statement.
There is no dialogue and no means of resolving disputes, no matter how minor. So it is not surprising when such disputes escalate into violence.
Foxconn does not confirm which of its plants supply Apple, but an employee told Reuters that the Taiyuan plant is among those that assemble and make parts for Apple's iPhone 5.
In June, about 100 workers went on a rampage at a Foxconn plant in Chengdu, in southwestern China.