Access to the popular social networking service Twitter and email service Hotmail was blocked across mainland China, two days before the 20th anniversary of the bloody Tiananmen Square crackdown.
While anniversary commemorations are illegal in China, crowds gather every year for a vigil in Hong Kong, a former British colony that enjoys greater freedom.
This year, calls for a re-evaluation of the protest movement have been published on the Internet, and this may have prompted the black-out.
Other Internet users reported not being able to access Windows Live, a service offered by Microsoft Corp. which also owns Hotmail, and the company's new search engine, Bing.com. Flickr, an online photo sharing service owned by Yahoo was also blocked.