Security forces with black masks and machine guns on the streets of China's capital are just the more visible side of a security clampdown in the country this month: there is also its secretive battle to control the Internet.
The heightened security comes ahead of a massive military parade Beijing will hold in the heart of the city next week to celebrate China's 60th anniversary of communist rule, an event the government hopes will showcase the country's development and go untarnished by security threats or shows of dissent. China's newest nuclear missiles will be included in the arsenal of weapons and equipment shown off in the parade, according to state-run media.
Security measures have included a crackdown this month on online tools that help users circumvent the "Great Firewall," the set of technical measures China uses to filter the Internet, according to providers of the tools.
"They put more resources into the blocking," said Bill Xia, president of Dynamic Internet Technology, which makes a widely used anti-censorship program called Freegate.
"It has been getting worse and worse this month," he said.
Many expatriates and savvy locals in China rely on Freegate as well as proxy servers and virtual private networks (VPNs) to bypass blocks that China places on Web sites like YouTube, Facebook and Twitter. But accessing some of those tools has become more difficult in recent weeks.
China has always blocked IP (Internet Protocol) addresses it believes are used by Freegate, which routes users' communication through foreign IP addresses to grant access to Web sites blocked in China. But this month it became more aggressive and began blocking a wider range of IP addresses, risking taking down unrelated targets in order to hit more Freegate users, Xia said. The moves have left most users unable to use the program, prompting Xia's company to ready an updated version of Freegate that will be available in a few days.
China also cranked up its efforts to stifle Freegate ahead of another sensitive date this year: the 20th anniversary of its bloody crackdown on student democracy protests in Beijing's Tiananmen Square in June 1989.
Measures China uses to limit access to certain Web sites include altering entries in the DNS (domain name system), which translates URLs like www.google.com into the numeric IP addresses used to relay information online, and resetting a computer's connection when it tries to visit a banned site. The country's police force also patrols the Internet for sensitive or pornographic content.
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.
Some VPN providers declined to comment for a news story for fear of drawing China's attention and potential restrictions on VPNs.
At least one Chinese city has adopted a further measure to monitor Internet traffic. The southern city of Guangzhou this month ordered Internet service providers to install "security monitoring" software on all servers and threatened punishment for failure to do so, according to government notices posted on the blog of one data center management company. Two such software programs, called Blue Shield and Huadun, were recommended in one of the government notices. Huadun's Web site says the program helps server owners remove illegal and pornographic content from their systems.
The software is meant to "create a favorable online environment" for China's National Day celebration next week, the government orders said. A representative of the data center company reached by phone said it put the orders on the blog for reference by clients and that the order applied only to Guangzhou.
Some of China's new security measures could remain in place long after the 60th anniversary celebrations, but others are likely to be lifted. China has long gone through cycles of blocking and allowing access to Web sites such as YouTube and Wikipedia, and updates to Freegate have repeatedly allowed the tool to bypass evolving government security measures against it.
Still, Chinese users have posted skeptical notes on Twitter about China's newest Internet controls. When asked if Twitter and Facebook would be unblocked after the National Day celebration next week, one user said they would not.
"Last year we had the Olympics, this year is National Day (which actually happens every year), and next year is the World Expo," the user wrote. "Actually, every year and every month and every day are sensitive."