Thomas Buergenthal believes in justice. This is not just heartwarming but remarkable because this is a man who has witnessed some of the most depraved behaviour in human history. As a child, he survived the Kielce Ghetto in Poland; he survived Auschwitz; he survived the death march to Germany, and he survived the Sachsenhausen death camp where he was ultimately liberated by the Polish army in 1945.

After being miraculously reunited with his mother after the war, Buergenthal settled with in the United States, where the second part of his life began - as a human rights lawyer and ultimately a judge at the International Court at the Hague. In his professional life, he has presided over cases of genocide and multiple human-rights abuses. That he felt able to do what he did given what he went through is also remarkable. But Buergenthal (now 77 and retired as a judge) felt it was his duty.

We sit in his London hotel lobby. Buergenthal is not feeling well and tells me he considered cancelling our interview. However, once he gets into his stride, his strength seems to return and he speaks fluently and persuasively in his softly spoken German-American English.

He explains his motivation: "Having survived I wanted to try to ensure that it didn't happen to other people. I trained as a lawyer partly because my father had and partly because I realised that I would never make a scientist or a physician. Gradually I realised that because of my language background and what I knew about the world, I could make a contribution to international law. I got my law degree in 1960 when things were beginning to stir - at around the time of the European Convention of Human Rights. I was fascinated to see what would happen. Could it prevent genocide? Before I knew it, I was an expert in the field, mainly because there weren't that many people who knew much about it."

Despite his dreadful experiences he maintains he has been fortunate in many respects - indeed his best-selling 2007 account of his Holocaust experience is entitled A Lucky Child. That he is not haunted by his awful experience - he improbably escaped death on several occasions - he puts down to his youth.

Things have got much better. What has happened in the past 20 years with the internet - it's impossible to keep abuses quiet these days

"At my age this was my normality. Everything was upside down in that world. It was terrible but it was also normal. That was my experience of the world at that time."

It was Labour peer Lord Janner, who as young British soldier came to the aid of Buergenthal and his mother in the confused aftermath of the war.

Buergenthal now finds that he no longer recalls the feeling of hunger and the terror he must have felt in those days. He also feels his recuperation was helped by the fact that he was transplanted to a normal, affluent America where he was given the opportunity to resume his education.

"It helped that no one knew anything about the Holocaust in those days. I went to college and law school and it was all behind me. It wasn't until the 1980s when people started asking me questions about it," he says.

He pauses for a moment before continuing: "It was strange. My uncle and aunt, who I lived with when I was at high school in New Jersey, knew a group of German refugees who left Germany in 1938 and 1939. Even the refugees never asked me any questions. I put it down to the fact that their English was bad but they still refused to speak German."

Buergenthal has always been able to speak about his experiences, But there are other things he is unable to do.

"It's strange because I can't go to movies about the Holocaust or watch television programmes. I tried to watch that film La Vita e Bella - some friends told me that it was a great movie. The first 10 minutes were nice but then I suddenly saw the railroad trucks I knew what was coming and I had to walk out."

However, it is precisely television and the institutions set up in the post-war world which he believes play a vital role in preventing human-rights abuses and ultimately genocide.

"Things have got much better. I believe that if we had the institutions we have today a lot could have been prevented in the 1930s - not once you reached the extermination phase but when Hitler was beginning to move against the Jews."

He adds: "What has happened in the past 20 years with the internet, TV and Twitter - you see some of these abuses happen in real time. It's impossible to keep things quiet these days. And tyrants must think about the fact that sooner or later they are going to end up in The Hague."

Buergenthal saw many of those accused of human-rights abuses while he was resident in the Dutch capital and on the Human Rights Committee at the United Nations. On occasion, he felt he had to absent himself.

He recalls one such case: "There was a Holocaust denier. One of the things he denied was that Auschwitz was a killing field. I did not feel that it was proper that I sit. By excusing myself I made a more effective contribution. I issued a statement that, as a survivor of Auschwitz and Sachsenhausen, whose father and grandfather were killed in Auschwitz and other death camps, I felt that I could not sit in this case. That withdrawal made an impact."

He also came face to face with his past when he presided on the El Salvador Truth Commission, investigating human rights abuses during the civil war in the Central American Republic.

"There were times when victims were speaking to the commission and I realised that I could finish their testimonies for them. I felt I couldn't let that experience affect my work. I had to be impartial but I struggled with it sometimes because I knew what it felt like to be a victim."

His perceived impartiality was also an issue when he was one of 15 judges who sat to examine the legality of Israel's security barrier. He was the only one of the 15 who dissented from the judgment that the fence was in contravention of international law. But sentiment played no part in his decision.

He explains: "We did not have enough facts to decide whether the whole wall contravened international law. Some segments certainly did. But if there is not sufficient evidence in a certain case one has the discretion not to arrive at a verdict, and this is what I decided.

"What was interesting was that my view was taken on by the Israeli Supreme Court and they actually pushed back the wall in certain instances. Israelis can be very proud of the independence of their court. It was, however, a big mistake for Israel not to participate in the case. It would have made our lives much easier."

Buergenthal sits back in his chair. The colour has returned to his cheeks. He chuckles as we shake hands: "I feel much better now. I was in bad shape this morning. It turns out all I needed was a diversion."