Beijing - Riot police have quelled two violent protests by thousands of people in southern and eastern China, in the latest of several recent clashes between ordinary residents and local authorities, officials and a rights group said on Friday.

Up to 30 people were detained for questioning after riot police broke up a protest by migrant workers demanding compensation for a teenage boy who was seriously injured after he climbed into a textile factory in the eastern city of Ningbo, police said.

"There are now 20 to 30 under investigation and being interviewed," a police officer in Ningbo's Xiangshan county told Deutsche Presse-Agentur dpa by telephone.

The officer said "mass rioting" broke out after factory workers seized the 14-year-old boy as a suspected thief and called police, apparently resulting in the boy jumping from a building as the police arrived.

"The reason why he jumped is still under investigation," he said.

The Xiangshan police officer said the boy was treated by a local hospital for serious injuries but was out of danger.

He said no police were injured in the clash with protestors, but the Hong Kong-based Information Centre for Human Rights and Democracy said 20 others were injured.

The Information Centre said some 500 migrant workers gathered outside the factory on Thursday, and thousands of onlookers swelled the crowd to about 10,000 before the riot police arrived.

It said another 50 people were injured and 20 detained after riot police clashed with thousands of protestors angered by an illegal fund-raising scheme in the southern province of Hunan on Wednesday and Thursday.

The government's official Xinhua news agency said a "chaotic petition to local authorities" on Wednesday and Thursday had blocked roads and railways in Hunan's Jishou city.

The protesters surrounded local government offices to demand intervention after a local real-estate firm failed to repay the loans and interest promised in its illegal fund-raising scheme, the agency said.

Some protesters and onlookers swarmed into the Jishou railway station on Wednesday evening but were "persuaded by officials to leave about an hour later," it said.

The protestors continued to block roads and the railway station on Thursday morning.

They dispersed after officials registered the petitioners' claims against the real-estate firm and detained company executives, the agency said.

The riots are among an increasing number of protests and violent incidents in recent years, reflecting simmering unrest over abuse of official powers and widespread cynicism towards the ruling Communist Party in many poor areas.

In one of the most serious incidents, up to 30,000 people rioted after the suspicious death of a teenage girl in the south-western province of Guizhou in late June.

The government later said a string of unresolved "social grievances" and encouragement by criminal gangs had sparked the rioting. (dpa)