Clashes erupted between hundreds of local people and migrant workers in a town in the Chinese province of Guangdong, police and reports say.
Violence broke out on Monday night in S--h--a--x--i township after a teenager was detained by local police, Z--h--o--n--g--s--h--a--n police said.
The fights were said to involve locals and workers from S--i--c--h--u--a--n province.
Authorities had taken away 20 suspects, a statement on Z--h--o--n--g--s--h--a--n city's official news portal said.
The area in southern China is home to many garment factories employing millions of migrant workers from across the country.
Z--h--o--n--g--s--h--a--n police said a teenager from C--h--o--n--g--q--i--n--g was detained by local police on Monday for allegedly beating a primary school student.
Police tied the boy up to restrain him and he also suffered injuries on his face, the statement added.
Family members and friends of the youth gathered outside the offices of local authorities and numbers swelled to about 300 on Monday night. Clashes began after people began throwing rocks, the police statement said.
Official statements said the situation was resolved swiftly and effectively, and rioting crowds were dispersed.
However, the Hong Kong-based Information Centre for Human Rights and Democracy said that at least 30 people were hurt and two police cars smashed. It said the incident was triggered by a fight between a S--i--c--h--u--a--n youth and a local man.
A South China Morning Post report said that the township was sealed off by authorities on Tuesday and residents were warned to remain indoors.
The clash is the latest in a series of recent incidents involving tensions between locals and migrant workers.
In July 2011 six people were jailed after a riot involving thousands of migrant workers in Z--e--n--g--c--h--e--n--g.
They threw stones at police, burned cars and ransacked government buildings after a confrontation between street vendors and security officials.
Migrant workers are often paid meagre wages and are not given the same rights as urban residents.
John Sudworth BBC News, S**h**a**x**i, Guangdong
The traffic is flowing again, but S**h**a**x**i, a clothing factory town just across the water from Hong Kong, is still tense.
The violence was serious enough for the authorities to call in hundreds of r--i--o--t police as reinforcements, many of whom are still here, sitting idly under the trees outside the government buildings.
A large crowd of migrant workers remains on the pavement opposite and shops and businesses have been boarded up and closed.
Towns like S**h**a**x**i have been at the heart of China's export-fuelled boom.
But the armies of cheap labour on which it has relied are growing increasingly restless over issues such as low wages, corruption and a lack of social rights.
Riots in Z--h--o--n--g--s--h--a--n, Guangdong: On Monday a fight broke out between a child from S--i--c--h--u--a--n and local children in the village of H**a**o**t**u in S**h**a**x**i Township, Guangdong Province. The argument over began over picking m--a--n--g--o--s and lead to blows. A local security officer was beaten in the scuffle. Migrant workers from S--i--c--h--u--a--n then began to gather, protest and riot. Officials kept silent about the incident for two days before announcing “the scene is clear.” Video of the clashes have been removed from all major websites. More on the incident from CDT and the BBC.