At least 12 Chinese labor activists have gone missing in recent days, in what sources close to them believe was a coordinated effort by authorities to silence the vocal group, most of them university students or recent graduates.

Authorities took away at least nine activists in Beijing, Shanghai, Guangzhou and Shenzhen late Friday, and three more activists were taken away Sunday in the city of Wuhan, the sources said.

The Ministry of Public Security, which oversees police forces across the country, did not respond to a faxed request for comment.