By Barbara Goldberg

NEW YORK (Reuters) - The death of World War Two concentration camp survivor and Nobel laureate Elie Wiesel was mourned on Sunday by admirers worldwide who honored his life-long fight for millions of Holocaust victims.

"My husband was a fighter," Marion Wiesel said in a statement. "He fought for the memory of the six million Jews who perished in the Holocaust, and he fought for Israel. He waged countless battles for innocent victims regardless of ethnicity or creed."

Wiesel, 87, died on Saturday at his home in New York City.

His wife was among mourners who attended a private funeral service on Sunday at the Fifth Avenue Synagogue on Manhattan's Upper East Side. A public memorial will follow at a later date, the Elie Wiesel Foundation for Humanity said.

Speaking to reporters outside the funeral, Abraham Foxman, former director of the Anti-Defamation League and a friend of Wiesel, said the world had lost a great moral voice.

"We the survivors lost the voice of memory. And, I personally have lost a very special friend," Foxman said.

Condolences from leaders around the world filled social media with memories of Wiesel demonstrating the triumph of goodness over inconceivable horrors.

His advocacy on behalf of Holocaust victims earned him the Nobel Peace prize in 1986. He told their story in his landmark book "Night," maintaining that "to forget the dead would be akin to killing them a second time."

Even as he received the Congressional Gold Medal at the White House in 1985, he rebuked U.S. President Ronald Reagan for planning to lay a wreath at a German cemetery where some of Hitler's notorious Waffen SS troops were buried.

'CONSCIENCE FOR THE WORLD'

In a tribute on Sunday evening, the lights of One World Trade Center's 408-foot (124-meter) spire in Lower Manhattan will display the blue and white colors of the Israeli flag.

New York Governor Andrew Cuomo said Wiesel was a legendary New Yorker, author and activist, and he added in a statement: "May Elie's memory forever be a blessing."

Officials at the Holocaust memorial in Israel said Wiesel was a voice for people everywhere in need of justice.

He fought for the rights of victims in Sudan's Darfur region, noted Rabbi Yisrael Meir Lau, chairman of Yad Vashem Council and former Chief Rabbi of Israel, as well as for Jews behind the Iron Curtain in the Soviet Union.

"Over time, Elie Wiesel became really a conscience for the world," Robert Rozett, director of Yad Vashem Libraries, told Reuters TV in an interview.

Wiesel's tenacity on behalf of Holocaust sufferers and the downtrodden was matched by his warmth and encouragement of loved ones, said his son Elisha Wiesel.

"My father raised his voice to presidents and prime ministers when he felt issues on the world stage demanded action. But those who knew him in private life had the pleasure of experiencing a gentle and devout man who was always interested in others, and whose quiet voice moved them to better themselves," he said in a statement.

"I will hear that voice for the rest of my life, and hope and pray that I will continue to earn the unconditional love and trust he always showed me," he said.

While the Romanian-born Wiesel was best known for his campaign never to let the world forget the Holocaust, one of his greatest rewards was working with students, including those at Boston University, where he was a religion and philosophy professor.

"What was most meaningful to him was teaching the innumerable students who attended his university classes," Marion Wiesel said.

Boston University said in a statement the school was heartbroken to have lost such an "iconic" teacher.

(Reporting by Barbara Goldberg; Editing by Tom Heneghan, Bernard Orr)