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.