China has kept a tight lid on content related to the 1989 Tiananmen Square crackdown, blacking out foreign TV reports about the 20th anniversary of the incident and expanding curbs on websites.

News reports about the bloody June 3-4 crackdown that ended seven weeks of pro-democracy protests at the square were abruptly cut off on Wednesday with screens periodically going black on the BBC and CNN in China, as they have all week.

China's censors also appeared to be blocking sites such as Microsoft's new search engine Bing, social networking service Twitter, photo-hosting website Flickr and others.

Attempts by AFP to access those sites resulted in an error page saying the site could not be displayed.

The sites join those regularly blocked by China at sensitive times, including YouTube, the BBC's Chinese-language service and press freedom group Reporters Without Borders (RSF).