Student tensions continue to rise in the lead up to the 20th anniversary of the Tiananmen Square crackdown, with information about the June 4 incident on the University of Hong Kong's democracy wall vandalised this week. Student union spokesman Arthur Cheng Fing-hin said the attacks were disturbing and came as the union was investigating the disappearance of archival material relating to the incident. On June 4, 1989, the central government ordered troops to put an end to student-led pro-democracy protests that had centred on Tiananmen Square. The crackdown left several hundred dead and sparked an outcry around the world. The students' union said a 'coffee-like substance' was used on a number of statements while the Chinese character for blood was written in red over information on an upcoming student vote on the June 4 crackdown. The attacks happened late on Wednesday night. In the April 14-16 poll, the union will ask students whether the central government should 'vindicate the 1989 democratic movement and be held accountable for the June 4 massacre'. 'We want the wall to be used by everyone to express their opinions, and we don't want to have to protect it in any way,' Mr Cheng said. 'It's actually really sad that anyone would even consider vandalising the democracy wall. 'We won't be taking any real precautions. We just want people to be tolerant and respect the views of others, which is what Hong Kong is all about.' The vandalism comes two weeks after the City University students' union council opposed distributing books at the public commemoration vigil marking the anniversary. The union's editorial board had proposed publishing 26,000 copies of a 60-page book that would review the Tiananmen Square crackdown from a human rights perspective. It intended to hand out the copies during the annual vigil in Victoria Park.