A news release posted on the Web site of Yuhuan County, which oversees Kanmen, said authorities were holding 23 people.
A woman who answered the telephone at Kanmen's public security bureau denied that workers broke into the police station or burned vehicles, saying they only gathered in the streets and shouted in protest. The woman did not give her name as is common with officials in China.
The Hong Kong-based rights group said the unrest in Kanmen was centered around a migrant worker who was beaten by a security guard while trying to get a temporary residence permit.
The violence comes just weeks after a crowd of 30,000 people in southwest China set fire to a police station, angry over what many believed was a cover-up of the death of a teenage girl by local authorities.
Such incidents are an embarrassment to officials, especially in the run-up to the Aug. 8 Beijing Olympics.
According to the rights group, when the worker went to police with a group of other workers to complain about the man who beat him, he was detained, triggering the protest in which hundreds of workers converged outside the police station, burning police cars and motorcycles and later throwing stones.
The report did not give any other details about the incident, including why the worker was beaten.
The chief of Yuhuan County's propaganda department, who gave his family name as Yan, denied that Zhang Zhongfu was beaten and said Zhang was drunk when he went to get his temporary residence permit.
"He knocked his head on the wall by himself," Yan said. He said Zhang has since been freed but gave no details.
A number for the Yuhuan County office that deals with migrant workers could not be given out to the public, a county phone operator said. Phones in the Kanmen government and the Yuhuan County public security bureau were not answered Monday.
Yan also said that more than 20 workers, not hundreds, later threw stones at police and police vehicles.
Also Monday, the official Xinhua News Agency said police in Guizhou province detained 100 people, including 39 members of local gangs, for involvement in last month's protest over the death of the student.
It quoted Peng Dequan, vice director of provincial public security, as saying they were still looking for other "gangsters" who were in hiding.
Authorities accused local gangs of fomenting the unrest and have urged offenders to surrender, Xinhua said.
