BEIJING—Chinese authorities detained or interrogated more than a dozen labor activists in a southern manufacturing hub this week, a sweep that underscores Beijing’s growing anxiety over worker unrest.

The activists, mainly based in the cities of Guangzhou and Foshan, were rounded up by police for questioning Thursday, according to some of the activists and their associates. They said as many as 19 people were taken in, though a precise number wasn’t clear because those detained work for different groups.

Fellow...