SAN FRANCISCO  A group of Canadian human-rights activists and computer security researchers has discovered a huge surveillance system in China that monitors and archives certain Internet text conversations that include politically charged words.

The system tracks text messages sent by customers of Tom-Skype, a joint venture between a Chinese wireless operator and eBay, the Web auctioneer that owns Skype, an online phone and text messaging service.

The discovery draws more attention to the Chinese government’s Internet monitoring and filtering efforts, which created controversy this summer during the Beijing Olympics. Researchers in China have estimated that 30,000 or more “Internet police” monitor online traffic, Web sites and blogs for political and other offending content in what is called the Golden Shield Project or the Great Firewall of China.

The activists, who are based at Citizen Lab, a research group that focuses on politics and the Internet at the University of Toronto, discovered the surveillance operation last month. They said a cluster of eight message-logging computers in China contained more than a million censored messages. They examined the text messages and reconstructed a list of restricted words.