Customers surf the Internet at an Internet cafe in Beijing in this June 20 file photo. Several Chinese Internet sites and parts of popular Web portals went offline Tuesday amid tightening controls that have already left mainland Web users without access to Facebook, Twitter and other well-known social networking sites.

Beijing ? Two more Web sites dedicated to social networking went offline in China on Tuesday amid tightening controls that have blocked Facebook, Twitter and other popular sites that offered many Chinese a rare taste of free expression.

China’s crackdown on social networking sites began in March, when Chinese Web users found they could no longer visit YouTube shortly after video appeared on the site purporting to show Chinese security officials mistreating Tibetans.

The blockages continued through the 20th anniversary of the Tiananmen Square demonstrations and the recent ethnic riots in Xinjiang, with homegrown and overseas micro-blogging and photo-sharing sites among those targeted.

Though cutting off access to sites can often be traced to a specific trigger — such as the June 4 Tiananmen anniversary — experts say the fact the sites are not coming back online shows the harsh measures are part of a long-term strategy to pare back the power of the Internet and silence some voices finding expression here.

“I am especially pessimistic about this fall and next spring,” said Wen Yunchao, a well-known Chinese blogger based in Guangzhou in south China. “I expect they will be more and more restrictive because they have yet to come up with a good way to manage the Internet. They are aware that it has this great power and they are afraid of it.”

Digu and Zuosa, two Chinese Web sites that offer micro-blogging services similar to Twitter, were both shut down for maintenance Tuesday, according to notices posted on their homepages. A Digu spokeswoman who would only give her surname, Zhang, said the site was offline so it could be moved to a new server. She said it would be down for at least a week.

“It’s a sensitive period, so we are not in a rush to re-open it,” Zhang said, adding that some Digu users had recently tried to post politically sensitive material to the site and that the company was having to censor such content. She wouldn’t give any specific examples.

Zuosa employees did not respond to an e-mail seeking comment about the site’s closure.

Wen, the blogger from Guangzhou, said having two sites close on the same day indicates pressure from authorities for them to shut down. He said the timing of the closures was probably related to the 10-year anniversary today of the banning of the Falun Gong spiritual movement.

“Maybe the Chinese government is concerned that the Falun Gong will use the occasion to spread some rumors or organize some kind of event via the Internet,” he said.

Also Tuesday, the technology channels of the popular Sina and Netease Web portals were shut for about six hours, apparently because they had posted news about a corruption probe without clearance from state censors.

China has the world’s largest population of Internet users, more than 298 million, and the world’s most extensive system of Web monitoring and censorship.

While the government claims the main targets of its Web censorship are pornography, online gambling, and other sites deemed harmful to society, critics say that often acts as cover for detecting and blocking sensitive political content.

The video site YouTube has been blocked in China since March — apparently because it contained video depicting harsh treatment of Tibetans by Chinese security forces. Twitter and the photo-sharing Flickr service were made inaccessible in June, just ahead of the 20th Tiananmen crackdown anniversary.

Facebook and Fanfou, a Chinese site similar to Twitter, were cut off after deadly ethnic riots erupted in China’s far western region of Xinjiang earlier this month.

The technology channels of China’s leading Web portals, Sina and Netease, could not be opened for several hours after both sites posted news about a Namibian probe into corruption allegations against Nuctech, a Beijing company that makes scanning equipment. The articles were deleted and the channels were online again by late Tuesday.

Xiao Qiang, director of the Berkeley China Internet Project at the University of California-Berkeley, said the closure of a whole channel is unusual.

“These two channels have a lot of readers … so their being shut down is a very big thing, more than shutting down a blog or something like that,” said Xiao. “My guess is … their chief editors are being punished.”