Mr. Wiesel first gained attention in 1960 with the English translation of “Night,” his autobiographical account of the horrors he witnessed in the camps as a teenage boy. He wrote of how he had been plagued by guilt for having survived while millions died, and tormented by doubts about a God who would allow such slaughter.

“Never shall I forget that night, the first night in camp, which has turned my life into one long night, seven times cursed and seven times sealed,” Mr. Wiesel wrote. “Never shall I forget that smoke. Never shall I forget the little faces of the children, whose bodies I saw turned into wreaths of smoke beneath a silent blue sky. Never shall I forget those flames which consumed my faith forever. Never shall I forget the nocturnal silence which deprived me, for all eternity, of the desire to live. Never shall I forget those moments which murdered my God and my soul and turned my dreams to dust. Never shall I forget these things, even if I am condemned to live as long as God himself. Never.”

Mr. Wiesel went on to write novels, books of essays and reportage, two plays and even two cantatas. While many of his books were nominally about topics like Soviet Jews or Hasidic masters, they all dealt with profound questions resonating out of the Holocaust: What is the sense of living in a universe that tolerates unimaginable cruelty? How could the world have been mute? How can one go on believing? Mr. Wiesel asked the questions in spare prose and without raising his voice; he rarely offered answers.

“If I survived, it must be for some reason,” he told Michiko Kakutani of The New York Times in an interview in 1981. “I must do something with my life. It is too serious to play games with anymore, because in my place, someone else could have been saved. And so I speak for that person. On the other hand, I know I cannot.”

There may have been better chroniclers who evoked the hellish minutiae of the German death machine. There were arguably more illuminating philosophers. But no single figure was able to combine Mr. Wiesel’s moral urgency with his magnetism, which emanated from his deeply lined face and eyes as unrelievable melancholy.

“He has the look of Lazarus about him,” the Roman Catholic writer François Mauriac wrote of Mr. Wiesel, a friend.

President Obama, who visited the site of the Buchenwald concentration camp with Mr. Wiesel in 2009, called him a “living memorial.”

“He raised his voice, not just against anti-Semitism, but against hatred, bigotry and intolerance in all its forms,” the president said in a statement on Saturday. “He implored each of us, as nations and as human beings, to do the same, to see ourselves in each other and to make real that pledge of ‘never again.’”

Mr. Wiesel long grappled with what he called his “dialectical conflict”: the need to recount what he had seen and the futility of explaining an event that defied reason and imagination. In his Nobel speech, he said that what he had done with his life was to try “to keep memory alive” and “to fight those who would forget.”

“Because if we forget, we are guilty, we are accomplices,” he said.

A year earlier, on April 19, 1985, Mr. Wiesel stirred deep emotions when, at a White House ceremony at which he accepted the Congressional Gold Medal of Achievement, he tried to dissuade President Ronald Reagan from taking time from a planned trip to West Germany to visit a military cemetery there, in Bitburg, where members of Hitler’s elite Waffen SS were buried.