One of Cuba's best-known dissidents, Oswaldo Paya, leader of the Christian Liberation Movement, died on Sunday in a car crash, government and opposition sources said.
Paya, 60, was traveling in eastern Granma province at the time of the accident, the details of which are not known, the sources said.
According to dissident blogger Yoani Sanchez, who broke the news on Twitter, Paya's death was confirmed by the bishop of Granma, Carlos Amador.
Paya's family was not immediately available for comment.
Another dissident died in the crash, and a Spaniard and Swede were injured, after the car left the road and hit a tree, government officials told Reuters.
In 2002, Paya spearheaded a petition campaign calling for a referendum on one-party rule and submitted more than 30,000 signatures.
The petition drive was rejected by the government, but Paya emerged as the leading advocate of peaceful democratic change in Communist-run Cuba.
Paya received the European Union's top human rights award in 2002, the Sakharov Prize, named after the late Soviet dissident Andrei Sakharov.
He was nominated twice for the Nobel Peace Prize by former Czech President Vaclav Havel.
A devout Catholic who was sent to a labor camp in the 1960s for his religious beliefs, Paya overcame intimidation and harassment to build Cuba's first nationwide opposition initiative.
"This is tragic for the family and the human rights and pro-democracy movement in Cuba," said Elizardo Sanchez, head of the illegal but tolerated Cuban Commission for Human Rights.
"Paya was considered the most notable political leader of the Cuban opposition," he said.
The petition campaign for "moderate reforms to Cuba's one-party state" was named the 'Varela Project' by Paya, after Felix Varela, an early 19th century Cuban priest who was one of the island's human rights leaders in the struggle for independence from Spain. The petition was rejected by President Fidel Castro's government as a U.S. plot to undermine his four-decade rule.
About 40 of Paya's grassroots activists, including his closest aides, were among 75 Castro critics arrested in a March 2003 crackdown on dissents and given jail sentences of up to 28 years.
They were released in 2011.
Paya, a soft-spoken, unassuming medical equipment engineer, continued to call for a national dialogue between Cubans, including members of the ruling Communist Party, to discuss a nonviolent transition to democracy.
He also had a strong following among Cuban exiles in the United States and elsewhere. In 2003, he visited the United States, where he was received by Secretary of State Colin Powell, before spending several days in Miami meeting with Cuban exiles.
"Through his leadership, Oswaldo inspired countless democracy advocates who have embraced and carried forth his vision of nonviolent political change," the Cuba Study Group, a leading Cuban-American exile organization, said in a statement.