China's latest efforts at tightening its control over the Internet - including the blocking of Gmail and web software that can bypass the censorship - have hampered the work of human rights activists, say groups based in the US.
"The Chinese government is specifically targeting the communications of human rights activists," said Phelim Kine, an Asia researcher with New York-based Human Rights Watch. "They've done this before, but this has been much more prolonged. It is much more intense."
In the past weeks, Internet users in China have reported greater difficulty with accessing Gmail, posting on microblogs that the service is slow or blocked. Several companies providing virtual private networks (VPNs), which can allow users to view sites and content blocked by Chinese Internet censors, have also reported access problems in the country.
Human rights activists use both Gmail and VPNs to communicate and access information over the web. But the recent blocking has isolated activists working in China, while making them fearful they will face subversion charges from the Chinese government, Kine said.
Experts say the increased censorship is tied to the Jasmine Revolution, an online call for the Chinese people to protest against the government sparked by the political unrest in the Middle East. The protest calls originated last month with a group of anonymous activists. But China has responded by detaining Chinese human rights advocates and deploying large police forces across cities to prevent any protests from occurring.
"The current campaign is sending a chill through the community of human rights defenders in China," Kine said. He added that the activists he knows in China are all currently detained. "The activist community is beleaguered, under-resourced and always under threat. Currently it's under lock and key. That's the state of play for activists in China."
Sharon Hom, the executive director for Human Rights In China, said communication between activist networks and their families in China has been unstable with the disruptions to Gmail and the VPNs. "Sometimes we just haven't been able to get through," she said. "It's made us concerned about the security of the communication."
China has the world's largest Internet population at 457 million, according to the latest official count. But information on the web is strictly controlled, with content deemed politically sensitive blocked or deleted from the sites.
VPNs have sometimes been the victim of censorship when highly political sensitive events occur like the 60th anniversary of Chinese communist rule back in 2009. But in the past only free services were blocked, while paid VPNs providers were unaffected, said Phil Blancett, the president of a VPN provider StrongVPN.com.
However, now paid VPNs such as StrongVPN and WiTopia are also being blocked. StrongVPN's customers include foreigners as well as businesses, Blancett said. "They blocked quite a few VPN provider websites from what we have seen," he said. "We don't understand the Chinese government's effort to crack down on VPN providers like ourselves."
China's tightening of control over the web could have greater effect on businesses operating in the country, said Mark Natkin, the managing director of Beijing-based Marbridge Consulting. He noted that in recent days, even commercial sites free of political content have been inaccessible, a sign that censorship is spilling over into business activities.
"I think as long as we continue to see the upheaval that we are seeing in North Africa and the Middle East, China will continue to be more sensitive than usual, so there will be a tighter control," Natkin said. He added that in the eye's of China's leadership, "The argument will always come down to the same thing: what sacrifices are necessary to maintain greater stability?"
But China could start to see some push back from the business and academic community over the increased censorship, Hom said. On Monday, Google accused the Chinese government of blocking Gmail after users had complained for weeks. The search engine company said the blocking was designed to make it look like the access problems were coming from Google.
"You are getting some companies like Google to go public by speaking out," Hom said. "Google is exercising leadership in this public arena, saying that this is a serious problem."
China's Foreign Ministry has denied Google's accusation. On Thursday, a Foreign Ministry spokeswoman said she had no comment on the blocking of VPNs because she did not fully understand the situation. But she added China's Internet is open and operates according to the law.