AS the advancing Red Army pushed into Germany, the SS guards at the Janwska concentration camp in Poland, realising they would be sent to the front if they had no prisoners left, decided to march westwards with their surviving inmates. By then, the 200 guards had just 34 prisoners left, out of an original 149,000.

Among those 34 was Simon Wiesenthal, an architect from the Ukraine whose entire family had been rounded up and sent to concentration camps by the Nazis. When he was finally liberated by American troops after reaching the Mathausen camp in Austria in May 1945, weighing just seven stone, he cried from loneliness. Then he dictated a list of 91 names of officials at the camps where he had been held. He was later to track down more than 70 of them.

He may have survived the death camps, but the experience was to both haunt and inspire him, compelling him to track down those responsible and bring them to justice. Wiesenthal, who died yesterday in his sleep at the age of 96, was to dedicate the next 50 years to pursuing those who planned and executed the Holocaust.

In total, he is believed to have brought 1,100 war criminals to justice, including Adolf Eichmann, one of the architects of the Final Solution. Tireless even in the face of official indifference, Wiesenthal worked from a tiny three-roomed office in Vienna with a staff of three, becoming the world's most prominent Nazi hunter, his name a byword for the dogged search for war criminals.

"Simon Wiesenthal was the conscience of the Holocaust," according to Rabbi Marvin Hier, dean of the Simon Wiesenthal Center. "When the Holocaust ended in 1945 and the whole world went home to forget, he alone remained behind to remember. He did not forget.

"He became the permanent representative of the victims, determined to bring the perpetrators of history's greatest crime to justice."

CHIEF Rabbi Sir Jonathan Sacks said: "Simon Wiesenthal was the voice of conscience of the post-Holocaust world. The sheer scale of the task he undertook was immense. His moral clarity and courage were unfailing. All those who fight hate are in his debt."

Wiesenthal was born in what is now Ukraine in 1908. He studied to become an architectural engineer, married Cyla in 1936, and settled in Lvov, in Poland.

But when Lvov was occupied by the Red Army after the Nazi-Soviet non-aggression pact in 1939, prominent Jews were purged. Wiesenthal's stepfather was jailed and his stepbrother executed.

Wiesenthal managed to save himself, his wife and his mother from deportation by bribing a Soviet commissar, but there was no hiding from the Germans when they invaded in 1941. His mother was sent to a death camp, while he and his wife were assigned to a forced labour camp, although they were later separated. In all, 89 members of his and his wife's families were killed.

After liberation, Wiesenthal began working for the War Crimes Section of the US Army, gathering and preparing evidence on Nazi crimes, as well as for a Jewish welfare organisation in Austria. He was also reunited with his wife, whose blonde hair had helped her pass as a non-Jew and escape the camps.

In 1947, he helped establish a centre in Linz, Austria, devoted to collecting information for use in war crimes trials, but as Cold War tension mounted, both the US and the Soviet Union began to lose interest in pursuing war criminals. Disheartened, Wiesenthal closed the Linz office in 1954, sending its files to the Yad Vashem Archives in Israel, except for one - the dossier on Adolf Eichmann.

Although many Nazi leaders had escaped justice by committing suicide - including Hitler, Goebbels and Himmler - and others had been tried at Nuremberg - Goering, Von Ribbentrop, Speer and Hess among them - many had escaped in the chaos of the final days of the war. One of the most prominent of these was Eichmann.

A technocrat who was head of the Gestapo's Jewish Department, Eichmann had been present at the Wannsee Conference of 1942, which decided on the extermination of European Jewry, and supervised the implementation of the Final Solution. In 1953, Wiesenthal had received information that Eichmann was living in Argentina, but the FBI believed he was in Damascus.

The confusion meant nothing was done in 1959, when Israel was informed that Eichmann was living in Buenos Aires under the name Ricardo Klement and working in a Mercedes-Benz factory. Agents for the Israeli secret service Mossad seized Eichmann in the street, and took him back to Israel, where he was tried and executed.

The capture of Eichmann encouraged Wiesenthal to renew his efforts to track down former Nazis, and he opened the Jewish Documentation Centre in Vienna. Collecting information from a worldwide network of sympathisers, German war veterans appalled at their countrymen's behaviour, and even former Nazis with scores to settle against their erstwhile comrades, Wiesenthal and his small staff amassed a huge store of documents and evidence.

ALTHOUGH some criticised him for not wanting to leave the past behind, he insisted that he wanted justice not vengeance. This phrase also lent itself as the title of a book which shed light on why he refused to abandon his mission. "Survival is a privilege which entails obligations," he wrote. "I am forever asking myself what I can do for those who have not survived. The answer I have found for myself is: I want to be their mouthpiece, I want to keep their memory alive, to make sure the dead live on in that memory."

High on his wanted list was Franz Stangl, the commandant of the Treblinka and Sobibor camps. After three years of painstaking pursuit, Stangl was located in Brazil and brought to justice in West Germany. He was sentenced to life in 1967 and died in prison.

Another priority was to challenge growing propaganda from Dutch Neo-Nazis that the diary of Anne Frank the 14-year-old girl who hid in an Amsterdam attic for two years before being found and sent to a concentration camp, was a fake. In 1963, Wiesenthal found Karl Silberbauer, a Gestapo officer then a police inspector in Austria, who confessed that he had arrested the schoolgirl.

In 1967, on a tour of the US to promote his memoirs, Wiesenthal revealed that he had found Hermine Ryan, nee Braunsteiner, who had supervised the killing of several hundred children at Majdanek, and who was then living as a housewife in Queens, New York. She was extradited to West Germany and sentenced to life in prison.

IN 1977, he established the Simon Wiesenthal Centre, a Jewish human rights organisation, in Los Angeles, and four years later the centre produced the Academy Award-winning documentary Genocide, narrated by Elizabeth Taylor and Orson Welles.

Wiesenthal's work brought him many awards, and he was a consultant on the feature films The Odessa File and The Boys From Brazil, but it also made him a target of threats and hate mail. After a bomb exploded at his front door in 1982, for which a German Neo-Nazi was jailed for five years, he was protected by armed policemen.

But Wiesenthal knew that his work would remain incomplete. Among his chief regrets were the failure to bring to justice the Berlin Gestapo chief Heinrich Muller and Auschwitz doctor Josef Mengele. The advancing years made it increasingly unlikely that surviving Nazis would be able to stand trial, and in his later years Wiesenthal came to accept that there would be few, if any, more trials. Mengele himself died in Brazil in 1978, and Muller, who has never been traced, would be 100 if he were alive today.

In 2003 Wiesenthal announced his retirement from Nazi hunting, devoting himself to answering letters, studying his files and working on his stamp collection. A few months later, in November, his wife Cyla died. But Wiesenthal knew that his work had taken on a greater importance than his own life. In a 1994 interview, he told the Washington Post: "The only value of nearly five decades of my work is a warning to the murderers of tomorrow, that they will never rest."