Cuba has lost a tireless advocate for freedom, a deeply religious family man who, in his quiet yet tenacious way, gave the Castro dictatorship its biggest challenge: a peaceful uprising of citizens demanding, through a signed petition, universal human rights, free markets, freedom of the press and association and an amnesty for political prisoners. It was called the Varela Project, and it caught the world’s attention.

Oswaldo Payá, who founded the Christian Liberation Movement, died Sunday in a car crash under suspicious circumstances. He was traveling with another dissident, Harold Cepero Escalante, also killed in eastern Cuba, about 500 miles from Havana. Two survivors traveling with them: Angel Carromero, deputy chairman of Spain’s ruling conservative Popular Party’s youth wing, and Jens Aron Modig, a Swede who chairs the Christian Democrat Youth League. Mr. Payá’s daughter says he had been pursued by a vehicle two weeks earlier, in a crash that he survived, and that the crash Sunday was another attempt to kill him. The international community should demand an independent investigation.

The Varela Project was met with disdain by the dictatorship and mistrust by some exiles. But even his most fervent critics in exile acknowledge he was a man of principle, who practiced his Catholic faith during the early years of the revolution and paid the price, sent to work camps and barred from university studies. His Varela Project so scared the Castros that after he delivered boxes of signatures to the assembly in 2002, the regime cracked down and jailed 75 Cubans in the opposition — 40 of them from Mr. Payá’s Christian movement.

As news spread on Sunday, Cuban blogger Yoani Sánchez noted that he “suffered the constant surveillance of his home, arbitrary arrests, repudiation rallies and threats . . . I never saw him break down, or yell, or insult his political opponents.” She titled her blog item “Rest in Freedom.”