He returned home from the United States to take part in the demonstrations, but was sent to prison for nearly two years for the role he played.
"The massacre in 1989 made a very deep impression on me," he said in an interview he gave to the BBC just a few months before he was arrested in 2008.
The activist once worked as a professor at Beijing Normal University, although he was eventually banned from teaching.
In 1996 he was again put away for speaking out about China's one-party political system, but this time he was sent to a re-education-through-labour camp for three years.
It was while there that he got married to Liu Xia.
Since then he has continued to discuss a range of taboo subjects, including criticising China's treatment of Tibetans.
http://www.bbc.co.uk/news/world-asia-pacific-11492131Reactions:
The Nobel Peace Prize should be awarded to people who "promote national harmony and international friendship, who promote disarmament and peace". Those are Mr Nobel's wishes.
Liu Xiaobo is a criminal who violated Chinese law.
It's a complete violation of the principles of the prize and an insult to the Peace Prize itself for the Nobel committee to award the prize to such a person.
In recent years, Chinese-Norwegian relations have maintained sound development, which is conducive to the two countries and two peoples' interests.
The Nobel Committee awarding Liu this prize, which runs contrary to the principle of the Peace Prize, will bring damage to two-way relations.
Sounds remarkably like the Republicans who opposed the decision of the Nobel Committee to award President Obama the Peace Prize last year.
US President Barack Obama
Last year, I noted that so many others who have received the award had sacrificed so much more than I. That list now includes Mr Liu, who has sacrificed his freedom for his beliefs.
By granting the prize to Mr Liu, the Nobel Committee has chosen someone who has been an eloquent and courageous spokesman for the advance of universal values through peaceful and non-violent means, including his support for democracy, human rights, and the rule of law.
As I said last year in Oslo, even as we respect the unique culture and traditions of different countries, America will always be a voice for those aspirations that are universal to all human beings.
Over the last 30 years, China has made dramatic progress in economic reform and improving the lives of its people, lifting hundreds of millions out of poverty. But this award reminds us that political reform has not kept pace, and that the basic human rights of every man, woman and child must be respected.
We call on the Chinese government to release Mr Liu as soon as possible.
I would like to offer my heart-felt congratulations to Mr Liu Xiaobo for being awarded this year's Nobel Peace Prize.
Awarding the Peace Prize to him is the international community's recognition of the increasing voices among the Chinese people in pushing China towards political, legal and constitutional reforms.
http://www.bbc.co.uk/news/world-europe-11499931