In preparation for the 20th anniversary of the Tiananmen Square massacre on Thursday, China has blocked access to Twitter, Hotmail, Flickr, MSN Spaces and several other web services, reports the Times Online.

The Internet crackdown, which began at 5pm local time on Tuesday, is part of a larger effort to minimize possible avenues of dissent before Thursday. Authorities also put a number of government critics under house arrest and censored both local and foreign media reports on the anniversary.

In 1989, tens of thousands of students and intellectuals gathered in Tiananmen Square to protest against authoritarian rule and to call for economic and democratic reform. On June 4, government tanks entered the square and cleared it of protesters, resulting in somewhere between 300 and 3000 deaths. The Communist Party has refused to apologize for the incident.