Authorities appear to have stepped up efforts to block other circumvention tools as well. China-based users of Hotspot Shield, another popular program that encrypts and reroutes online activity, have had problems accessing the program's Web site since last month, a representative of developer AnchorFree said in an e-mail.

China last month also started blocking the Web site of Blacklogic, a VPN provider, a company representative said, though the Web site can currently be accessed from China. The company had to switch to a new tunneling protocol when some users recently became unable to connect to any servers, the representative said.

"I'm unable to tell you with a 100 percent guarantee what [technical] measures are taken in China to interfere with our service, but these measures are being taken," the representative said.

Not all VPN providers appear to have been affected. China has mainly blocked free VPNs and proxies while allowing similar paid services, a representative of VPN provider 12vpn said in an e-mail.

Accessing blocked Web sites is fairly easy in China and many users do so through free Web-based proxies. Most VPN users in China are expatriates, but more local Chinese may be signing up as well. 12vpn and other tool providers said their number of China-based users rose after early July, when China blocked Facebook and Twitter.