UPDATE 2-China blocks Twitter service ahead of anniversary

* Chinese censors block Twitter, Hotmail, Flickr

* Moves come before 20th anniversary of protest crackdown (Adds Microsoft confirmation, comment, paragraph 9)

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."