“He was a dissident even among dissidents,” Yu Jie, a friend and biographer, said. Mr. Yu now lives in the United States.

He added, “Liu Xiaobo was willing to criticize himself and reflect on his actions in a way that even many activists in the democracy movement can’t.”

Mr. Yu recalled the first time Mr. Liu spoke to him over the phone, in about 1999. “He said, ‘I’ve read your book, and there’s a lot I disagree with,’ ” Mr. Yu said. “He criticized me for about half an hour.”

Mr. Liu started out as a notoriously abrasive literary critic in Beijing in the 1980s. He was called a “dark horse” who bridled at intellectual conformity, even in the name of reform. But he was increasingly drawn into political questions as Deng Xiaoping, the Communist leader, resisted matching economic liberalization with political transformation.

In 1989, he was a visiting scholar at Columbia University when students in Beijing occupied Tiananmen Square to demand democratic changes and an end to party corruption. He returned to Beijing to support the protests. He later described that time as a turning point, one that ended his academic career and set him irrevocably into a life of political opposition.

Mr. Liu’s sympathy for the students was not unreserved; he eventually urged them to leave Tiananmen Square and return to their campuses. As signs grew that the Communist Party leadership would use force to end the protests, Mr. Liu and three friends, including the singer Hou Dejian, held a hunger strike on the square to show solidarity with the students, even as they advised them to leave.

“If we don’t join the students in the square and face the same kind of danger, then we don’t have any right to speak,” Mr. Hou quoted Mr. Liu as saying.