Workers clashed with police at a Covid test kit factory in China, videos posted on social media showed.
The protest started on Saturday after a sudden announcement of layoffs and disputes over wages in the south-western city of Chongqing.
Some videos were filmed at Zybio, a company specialising in in-vitro diagnostic reagents and equipment.
China's end to its zero-Covid policy has seen requirements for negative test results dropped across the country.
In one video, hundreds of workers could be seen gathering outside the factory, some of them shouting: "Give us our money back!"
Boxes of antigen test kits could be seen strewn on the ground.
Some videos suggest later at night some protesters threw chairs, crates and traffic cones at a group of riot police deployed at the scene, forcing the officers to retreat.
"It is only right and proper that you need to pay money back if you owe others, this is the road of demanding unpaid salary," captions on the videos read.
Posts online said the protest started on Saturday morning, when workers were told they were effectively laid off and that salaries would not correspond to those promised by the workers' agents.
Although political demonstrations are rare in China, protests over labour issues and demonstrations targeting specific companies occur frequently.
China's rigid zero-Covid policy eventually led to widespread protests in November and the government announced a liftig of most restrictions soon after.