In October 2010, while serving his sentence at Jinzhou Prison, in northeastern China, Liu was named the winner of the Nobel Peace Prize for "his long and non-violent struggle for fundamental human rights in China."

Liu's absence from the Nobel ceremony was marked by an empty chair.

"We now have to come to terms with the fact that his chair will forever remain empty," Reiss-Andersen said.

"At the same time, it is our deep conviction that Liu Xiaobo will remain a powerful symbol for all who fight for freedom, democracy and a better world."

Messages of solidarity and sadness

World leaders and rights campaigners were quick to react to the news of Liu's death with messages of grief and sharp condemnation.

The White House called Liu a "political prisoner," and offered "heartfelt condolences" to Liu Xiaobo's wife, Liu Xia, and his family and friends.

Liu was first jailed for his role in the 1989 pro-democracy movement after the military crackdown in Beijing's Tiananmen Square -- and later for petitioning for political reform and co-writing a paper on policy toward Taiwan that was at odds with the government stance.

His most recent conviction, in December 2009, stemmed from his co-authorship of Charter 08, a manifesto calling for political reform and human rights in China.

Taiwan's President Tsai Ing-wen, one of the few political leaders to openly call for Liu's freedom throughout her time in office, called on China to help realize Liu's "Chinese dream" by implementing political reforms and allowing citizens "the natural right to democratic freedom."

The leader of the self-ruled island regarded by Beijing as a renegade province wrote that Liu "has no enemies, because democracy has no enemies."

Democracy advocates in Hong Kong, where Liu's activism continues to resonate strongly, vowed to "continue our fearless non-violent protests."

A spokesperson for the German chancellor, Angela Merkel, paid tribute to "a brave fighter for civil rights and freedom of expression."

At a press briefing Friday morning, Japan's Chief Cabinet Secretary Yoshihide Suga expressed "sincere condolences on the death of Liu Xiaobo who devoted his life to freedom and democracy," adding that "freedom, human rights and the rule of the law are universal values in the international community, and we think it is important they are secured in China as well."

In a statement, US Secretary of State Rex Tillerson said Liu "dedicated his life to the betterment of his country and humankind, and to the pursuit of justice and liberty," and urged Chinese authorities to release Liu's wife from house arrest and allow her to travel outside China.

Liu's wife, Liu Xia -- an artist and a poet -- has been under house arrest since Liu's Nobel victory.

With her communication with the outside world almost completely cut off by the government, Liu Xia has been suffering severe depression, according to friends, especially after authorities sentenced her brother to 11 years in prison over what supporters call trumped-up charges of business fraud.

The UN High Commissioner for Human Rights Zeid Ra'ad Al Hussein echoed calls for her release, asking that the Chinese authorities give her the right to travel.

"Liu Xiaobo was the true embodiment of the democratic, non-violent ideals he so ardently advocated," Zeid said. "Despite the imprisonment and separation from the wife he adored that could have (fueled) anger and bitterness, Liu Xiaobo declared that he had no hatred for those who pursued and prosecuted him."

'Cruelty and callousness'

In the Chinese government's first official statement, Foreign Ministry spokesman Geng Shuang said the authorities "made all-out efforts to treat him out of humanity and in accordance with law" after Liu was diagnosed with liver cancer.

Geng said Liu was convicted of violating Chinese law and criticism of China's handling of the case was an "improper" intrusion into the country's domestic affairs.

Many of Liu's friends and supporters took to social media to contest the government stance.

Chen is the author of "The Barefoot Lawyer: A Blind Man's Fight for Justice and Freedom in China."

Chinese human rights lawyer and scholar Teng Biao, who fled the country in 2014, tweeted: "Liu Xiaobo has died. His love, courage and strength will never die."

Ai Weiwei, another Chinese dissident and perhaps the country's most famous living artist, tweeted: "Liu Xiaobo is gone -- rest in peace. We are here -- with Xiaobo."

Some of the sharpest condemnation of the Chinese government came from Human Rights Watch. The government's "arrogance, cruelty, and callousness are shocking -- but Liu's struggle for a rights-respecting, democratic China will live on," said the organization's China director Sophie Richardson.

Salil Shetty, secretary-general of Amnesty International, called Liu "a man of fierce intellect, principle, wit and above all humanity."

"Despite enduring years of persecution, suppression and imprisonment, Liu Xiaobo continued to fight for his convictions," Shetty said.