A strike at a Chinese factory that manufactures shoes for Adidas and Nike has spread to a sister factory in a neighbouring province, as thousands of workers gathered to protest against what they said were unfair pay and benefits.

About 2,000 workers clocked in on Monday, but did not work, at the Yue Yuen factory complex in Jiangxi province, southern China, joining at least 10,000 employees at another Yue Yuen factory complex in Dongguan, Guangdong province, who have been on strike since 14 April.

Up to 30,000 employees have stopped working in the strike – China's largest in recent memory, according to the New York-based non-governmental organisation (NGO) China Labour Watch.

"The issue that [the workers] are concerned about is very widespread," said Geoff Crothall of the Hong Kong-based China Labour Bulletin, another NGO. "In this case, at least the company was paying something, it just wasn't the full amount. In other cases we've seen, workers are getting nothing at all.

"A lot of this has to do with the fact that a lot of factories are closing down or relocating, or changing ownership ... Five years ago, [strikes] were all about wage increases. But the focus of workers' concerns now is very much on what happens if the factory closes down. What kind of payments do we get? Do we get the social insurance that we're legally entitled to?"

The Dongguan complex, which is operated by the Taiwan-based Pou Chen Group, has at least 40,000 employees and produces footwear for Reebok, Nike, and more than 20 other brands. The Jiangxi complex mainly produces shoes for Adidas.

Chinese authorities have deployed riot police and warned strikers against gathering at the factory. Last week the state newswire Xinhua reported that dozens of workers had been taken away by police. According to the China Labour Bulletin four workers were taken to hospital on the first day of the strike. Chinese state media has claimed that no one was injured.

"We're continuing the strike," a worker in Dongguan surnamed Zhou told Reuters on Monday. "We swiped our cards and then went back [home]. The other production lines in the same network are striking too."

Images online show massive crowds gathered in front of an eight-storey building, many apparently taking pictures with their mobile phones. In one, protesters carry a large banner, which reads: "Give me back my social insurance, give me back my housing benefits!"

Yue Yuen's spokesman, George Liu, told Chinese media that the firm had offered to raise workers' living allowance at its southern Chinese factories by 230 yuan (£22) a month, beginning on 1 May. It also promised to introduce a social security benefit plan next month. On Tuesday, Yue Yuen's management could not be reached for comment.

One worker, Xiang Feng, 28, said at least 80% of the workers were likely to refuse the offer. "Workers may end up with a take-home salary almost unchanged or maybe even lower than before," she told Bloomberg.

The government of Guobao, the city in which the factory complex in Jiangxi province is located, has urged Yue Yuen to assuage the workers' concerns without bending the law. "Company and worker representatives are urged to strengthen communication and consultation ... and guide the rational expression of the aspirations of the workers," it said on Monday.

China's wages have risen in recent years and international companies such as Adidas and Nike have begun moving their manufacturing operations to lower-cost countries including Vietnam.