Tens of thousands of workers were on strike Wednesday at a factory in southern China which makes shoes for Nike and Adidas, rights groups and a worker said.

More than 30,000 employees at the Yue Yuen Industrial (Holdings) factory in Dongguan city have been striking for several days in protest at unpaid social insurance payments, said US-based China Labour Watch, adding that police had beaten and detained several protesters.

China is facing labour unrest as its economic growth slows and as factories in its southern manufacturing heartland report a shortage of workers, prompting rising demands from staff.

Yue Yuen says on its website that it produces shoes for foreign brands including Nike, Adidas, Puma and New Balance. The factory at Dongguan in Guangdong province is partly Taiwanese-owned.

A spokesperson at Yue Yuen's office in Hong Kong was not immediately available for comment, while Puma denied having a working relationship with the Dongguan plant in a statement to AFP.

The strike comes as the massive factory traditionally gears up production ahead of the summer season in Europe and the US.

"The workers are still striking, and the numbers have probably increased," said Dong Lin, a worker at the Shenzhen Chunfeng Labour Justice Service Department, a rights organisation in close contact with the workers.

"They are demanding unpaid social benefits."

He estimated 40,000 workers were involved at the plant, adding that no deal had been reached. "The factory has released a notice saying it will dismiss the workers if they continue," he said.

Factory authorities have promised workers they will make the welfare payments some time before the end of 2015, a female employee told AFP, declining to be named due to fear of arrest.

But workers were not satisfied with the offer, she added. "The factory could just leave in the middle of next year, and we might end up without welfare payments."

She added that police had beaten and detained a handful of protesters earlier this week, and armed police were still stationed outside the factory gate even though the mood had calmed.

"No one is willing to stand out and lead the protests, because we are scared that the factory will take revenge against us later," she said, adding: "Local media does not dare to report our strike."

China Labour Bulletin, a Hong Kong-based rights group, recorded 202 labour disputes in the country during the first quarter of 2014, mostly in the manufacturing sector.

This represented a year-on-year increase of more than 30 percent.