Nobel laureate Liu Xiaobo, one of China’s most prominent political prisoners, died Thursday of multiple organ failure after a battle with liver cancer, according to Chinese officials. He was 61 years old.

The dissident died in Chinese custody days after the government reportedly denied his request to travel overseas for medical treatment. World leaders and supporters advocated for Liu’s release but Beijing officials denied the request, saying Liu was too ill to travel and was already receiving the best possible care. Two Western doctors visited Liu on Sunday and said they believed he could be safely moved for treatment, according to CNN. But officials in Beijing resisted, one telling reporters that “we hope relevant countries will respect China’s judicial sovereignty and not use a so-called individual case to interfere in China’s internal affairs.”

The activist died at the First Hospital of China Medical University with guards nearby, according to government reports.

“After multiple treatments, Liu Xiaobo’s condition continued to deteriorate,” the Bureau of Justice for the northeast Chinese city of Shenyang said in statement. “On July 10 he entered a state of rescue and intensive care, and on July 13 he died due to multiple organ failure after attempts to save him failed.”

Liu was diagnosed with cancer late in late May while serving an 11-year sentence for “inciting subversion of state power” after co-authoring a pro-democracy manifesto, titled “Charter 08.” The document called for more freedom of expression, human rights and an independent judiciary in China.

The activist routinely criticized the Chinese government for its communist, single-party rule, and fought for greater human rights.

Liu was previously imprisoned for 21 months in connection to China’s 1989 Tiananmen Square pro-democracy protests, where he helped negotiate the safe passage of students.

Liu was awarded the Nobel Peace Prize in 2010 while serving during his most recent prison sentence. He is the first Nobel laureate to die in state custody since Carl von Ossietzky in Nazi Germany in 1938, The Guardian reported.

Tributes poured out after news of Liu’s death surfaced. Many of Liu’s supporters also blamed the Chinese government for what they consider to be his premature death.

“The death of Liu Xiaobo today from a virulent cancer contracted while serving an 11-year prison sentence will forever be a black mark marring China’s reputation under international law and global human rights standards,” the free speech advocacy group PEN America said in a statement emailed to the NewsHour.

“[Liu] should never have spent a single day in jail, an injustice made even more tragic by the fact that his cancer was not diagnosed until it reached terminal phase,” Salil Shetty, secretary general at Amnesty International, said in a statement last week.

Human Rights Watch also called out the Chinese government for what it called “arrogance” and “cruelty.”

“But Liu’s struggle for a rights-respecting, democratic China will live on,” Human Rights Watch China Director Sophie Richardson said in a statement.