Mainland authorities who have long sought to limit access to information have reinforced the so-called Great Firewall of China, internet firms say, frustrating businesses and raising concerns among activists.
The firewall - the country's system of internet limits and restrictions - has stepped up its targeting of virtual private networks (VPNs), commonly used to bypass controls on websites the government considers a threat.
By using proxy servers located overseas and data encryption, VPNs let users reach sites blocked because of their content or sensitivity, among them Facebook and Twitter, and are also vital to firms by enabling secure communication.
At the same time they have compounded authorities' difficulties in seeking to shape public opinion and limit independent social organisation in the country's online community of 500 million, the world's largest.
Now web users are complaining of VPNs being inaccessible or quickly going down once accessed, while speeds have slowed to a crawl. A senior manager of a major French company said he had encountered a serious problem when using his company's VPN in business travel on the mainland last week.
"I could get connected with the VPN and log in with a password, but from there I couldn't go anywhere. After a while the connection was reset," he said.
"I tried it many times and even consulted the IT department in Paris, but they couldn't solve the problem. They told me the problem must be China's internet firewall."
The manager said his company was involved in some of the most costly infrastructure projects in China. Without the VPN, their sensitive business information could be easily stolen.
In a message to its users, VPN provider Astrill said the firewall system now had the "ability to learn, discover and block VPN protocols automatically".