Chinese dissident Fang Lizhi, a key figure in the pro-democracy movement behind the 1989 Tiananmen Square protests, has died in the United States, a fellow activist said Saturday. He was 76.

Fang, an internationally renowned professor of astrophysics, was granted refuge at the US embassy in Beijing for one year after publicly supporting the protests and he was forced into exile in 1990.

Wang Dan, a leader of the 1989 demonstrations who also lives in exile in the United States, told AFP in an email that Fang had died early Friday at home in Tucson, Arizona.

"This man was a treasure to China, but there was no place for him to die in his own country. He had to die in exile," Wang said in a tribute posted on Facebook.

Wang said Fang's wife informed him of the sudden death of the democracy and human rights activist, who was nicknamed "China's Sakharov" in a comparison to Soviet physicist and fellow civil liberties advocate Andrei Sakharov.

Fang came to prominence when he was dismissed as vice president of the University of Science and Technology of China in Hefei and was expelled from the Communist Party of China in January 1987, after making speeches urging reform.

Wang wrote that Fang had inspired the 1989 generation and awoke in the people their yearning for human rights and democracy.

"During all these years I spent in prison and then in exile, we stayed very close. I considered him to be part of my family," said Wang, himself considered a Tiananmen hero and sent into exile in 1998 after nine years in prison.

In open letter coinciding with Fang's death, Wang, hunger striker Wuer Kaixi and four others who participated in the 1989 Democracy Movement appealed for the Chinese government to let them return home.

"We hope that you will follow the currents of history, abandon the old practices of not allowing people to return to their country because of differences in political views, and use various effective means to permit us to return to China to visit," said the letter.

The Tiananmen protests, best known by the image of a lone man defiantly attempting to stop the advance of a column of tanks, were crushed by the government.

Hundreds, perhaps thousands, are believed to have died when China's leaders sent in tanks and soldiers to clear the enormous square in Beijing on the night of June 3-4, 1989, bringing a violent end to six weeks of protests.

An official verdict after the protests called them a "counter-revolutionary rebellion," although the wording has since been softened.

Fang became a leading figure in the student movement in 1986-87, but the Chinese authorities who fired him from his university post said his speeches had incited unrest.

His actions, which would culminate in the Tiananmen protests three years later though he never formally participated in them, led him and his wife to take refuge at the US embassy.

American diplomats refused to hand them over to the authorities, and Fang stayed at the US mission for a year before China authorized him to leave the country.

Having eventually settled in the United States, Fang became a professor of physics at the University of Arizona in Tucson.

The son of a railway worker, Fang was born on February 12, 1936 in Beijing. While in China, he openly declared he did not believe in Marxism, frequently criticized the Chinese government and questioned the legitimacy of the Chinese Communist Party.

Government officials considered him a traitor and denigrated him in official media.

Fang gained international notoriety when the Chinese government barred him from attending a Texas-style barbecue hosted by US president George H.W. Bush at a Beijing hotel in February 1989.

The incident sparked a small diplomatic crisis between Washington and Beijing, with Bush expressing regret over the matter.

Annual protests in memory of the Tiananmen demonstrations remain a taboo topic in Beijing.

China attempts to block any public discussion or remembrance of the events by hiding away key dissidents in the run-up to June 4 each year, taking them into custody or placing them under house arrest, friends and activists say.

Rights groups, including New York-based Human Rights Watch, have repeatedly called for China to be held accountable for its past and present actions, but Beijing consistently reiterates its position that the matter is closed.