China has blocked access to many social networking and other Internet services across the mainland as the government braces for the 20th anniversary of the Tiananmen Square pro-democracy protests.
Users have posted angry messages on available sites after the increasingly popular Twitter was blocked Tuesday, just two days before the anniversary of the Chinese military's bloody suppression of the protests in Beijing on June 4, 1989.
China has never allowed an independent investigation into the crackdown, and the number of victims remains a taboo subject.
Chinese authorities have stepped up security around Tiananmen Square to prevent any demonstration marking the event.
The Foreign Correspondents' Club of China says that police have prevented at least four TV crews from entering the square and harassed a reporter who interviewed the mothers of the victims.
The group says on its Web site Tuesday that it has received reports of the authorities harassing journalists and their sources. Foreign correspondents cite reports of university students being interrogated after talking to journalists. They also say at least one journalist received a warning from the Ministry of Foreign Affairs.
The Foreign Correspondents' Club of China also says it has heard reports of tighter censorship of Web sites, publications and TV channels, such as the BBC.
Some information for this report was provided by AFP