The filtering appeared to occur for searches done both in and outside of China. A search for "Tiananmen" returned images of tanks rolling into Beijing's central square in 1989 to crush pro-democracy protests if the search was written in English or in traditional Chinese characters, which are used in Hong Kong and Taiwan.

But those images did not appear for the same search done in simplified Chinese. Stately photos of the Tiananmen gate leading to Beijing's old imperial city instead filled the page.

Search results appeared to be missing for other politically charged words in simplified Chinese as well. Searching for "Falun Dafa," a name for the Falun Gong spiritual movement banned as a cult in China, turned up the movement's Web page if performed in English or traditional Chinese, but not when done in simplified Chinese.

Like Google's local search engine, Bing also appeared to filter out some sensitive results in any language when used from an IP (Internet Protocol) address in China. The Falun Gong Web site could not be found using traditional characters in China.

China has stepped up efforts to control content on foreign Web sites in recent weeks. Bing, Twitter, Flickr and other sites were blocked for the June 4 anniversary of the Tiananmen crackdown in an apparent attempt to prevent public commemoration or other disturbances.

China appeared to block Google's English search engine and other Google sites for over an hour Wednesday night, following state media criticism of the company last week for serving pornographic search results.

The Internet is heavily patrolled in China by police and by domain owners who fear punishment if they allow discussion of sensitive issues to appear on their Web sites. YouTube and some foreign news sites are blocked in China, and others including Wikipedia have been blocked before.

Microsoft did not immediately reply to a request for comment.