UPDATE 1-China blocks Twitter service ahead of anniversary

* Chinese censors block Twitter, Hotmail, Flickr

* Tightening ahead of 20th anniversary of protest crackdown (Adds TV, other Internet blocking, paragraphs 3-4, 8-9, 13)

By Lucy Hornby

BEIJING, June 2 (Reuters) - Access to the popular social networking service Twitter and email service Hotmail was blocked across mainland China on Tuesday afternoon, two days before the 20th anniversary of the bloody Tiananmen Square crackdown.

Before dawn on June 4, 1989, tanks rolled into Tiananmen Square in the heart of Beijing to quell weeks of protest by students and workers. China has never issued a death toll from the crushing of what it calls a "counter-revolutionary" conspiracy.

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.

Indignant Twitter users filled chatrooms with protest after access to Twitter was denied shortly after 5 p.m. (0900 GMT) on Tuesday.

"The whole Twitter community in China has been exploding with it," said Beijing-based technology commentator Kaiser Kuo. "It's just part of life here. If anything surprises me, it's that it took them so long."

By evening, residents of some cities in the southern province of Guangdong reported that television stations from neighbouring Hong Kong had also been blocked.

Other Internet users reported not being able to access Windows Live, a service offered by Microsoft Corp. (MSFT.O: Quote, Profile, Research) which also owns Hotmail, and the company's new search engine, Bing.com. Flickr, an online photo sharing service owned by Yahoo (YHOO.O: Quote, Profile, Research) was also blocked.

"This is so frustrating. Now I feel China is exactly the same as Iran," said a financial professional and avid Twitter user in Shanghai, referring to Iran's May ban on popular social networking site Facebook.

Twitter is an Internet-based text message service that allows users to post updates -- called "tweets" -- of no more than 140 characters.

Users in Beijing reported accessing the service without difficulty earlier on Tuesday, and even successfully searching potentially sensitive words such as "Tiananmen".

But signals from foreign television news channels cut out for minutes at a time in foreign compounds and hotels in Beijing and Shanghai as censors blacked out reports on the anniversary.

A guest in one hotel in Shanghai said that news transmissions were repeatedly replaced by public service warnings against smoking.

While professional and urban Chinese often use foreign Internet tools, including Twitter, Hotmail and Facebook, the vast majority of Chinese use similar domestic services that are carefully monitored for any sign of content deemed subversive.

Access to video-sharing site YouTube, owned by Google (GOOG.O: Quote, Profile, Research), has been blocked in China since March, after overseas Tibetan groups posted graphic footage of China's crackdown on protests by Tibetans in 2008. (Additional reporting by George Chen in Hong Kong; editing by Tim Pearce)