In London this week, Professor Judea Pearl tells why grief for his son, executed by a former LSE student in Pakistan, moved him to fight for reconciliation between Muslims and Jews.

When Professor Judea Pearl says that he 'wants revenge' and that 'it's a natural, primitive feeling', you can understand why. To lose an only son in the tragic way he did - Daniel, 38, a correspondent for the Wall Street Journal, was abducted and beheaded in 2002 in Pakistan by Islamic extremists who also filmed his murder, would destroy most fathers. But the enlightened 'revenge' that Professor Pearl is pursuing is not what you'd expect. 'Hatred took my son,' he says, 'and hatred I shall fight for the rest of my life.

When the American consulate phoned us with the devastating news that Danny was dead, my first response was amazement, amazement that the blood still flowed in my veins, that I could still lift an arm. A knife had been plunged through my heart and I was surprised I could still move my fingers. And yet within days, he and his wife, Ruth, had decided to harness the raw energy released by their anger and grief to establish a foundation promoting understanding between Muslims and Jews. 'We couldn't cope with the idea that Danny's service to humanity was terminated and we wanted to create a legacy that embodied his gentle, golden spirit. You see, our son was born with an awful affliction - a belief in the goodness of humanity, and a total absence of malice.'

The Daniel Pearl Foundation attempts to build 'a coalition of the decent' and seeks to empower the voices of reason in the Muslim and Jewish communities. Pearl, 70, a UCLA professor of computer science, found a partner in Professor Akbar Ahmed, chair of Islamic Studies at American University in Washington DC, and formerly the Pakistani ambassador to Britain. These two grandfathers - a Jew and a Muslim - host open forums in which the trenchant grievances of their respective faiths are aired and reconciliation is attempted through dialogue. So far, they have taken their healing road shows to 13 cities around the world, including London.

This week, Professor Pearl was in the capital to speak at a fund-raiser, hosted by the Council for Christians and Jews at Crosby Hall in Chelsea, where he topped a heavyweight bill that included the Archbishop of Canterbury. The following morning, jet-lagged after his long-haul flight from Los Angeles and drinking beer - 'for me it's still night-time,' he smiles, eyes sparkling mischievously - he gave his first interview in Britain, telling the Evening Standard about his extraordinary journey.

'At this stage of my life, I hoped to be slowing down, but I've become a dedicated social activist working 23-hour days, trying to focus the energy of thousands of people around the world who never knew my son but were intensely moved by his death,' he says. He is a powerfully built, stocky man, articulate and feisty, but funny, too, and he oozes charisma. When I ask who is playing his part in the Hollywood film ‘A Mighty Heart’, due out next year and starring Angelina Jolie as Daniel's French wife, Mariane (on whose memoir the film is based), he laughs. 'Me? I forgot his name. It's a minor role.'

The family's traumatic ordeal began on 23 January 2002, when Mariane, then pregnant, phoned from Pakistan, concerned because Daniel - who was investigating links between Pakistani militants and British shoe bomber Richard Reid - had not shown up. Professor Pearl began pacing, acutely worried, then he frantically hit the phones, calling the State Department, the FBI, the embassy, and anyone else who could help. Two days later, a picture of Daniel with a gun to his head was released by his kidnappers.

'My first response was tremendous glee that Danny was still alive.' He pauses, 'and then came tears. To see my son, whom I loved with every fibre of my being ...' he breaks off. 'Daniel has two sisters who adore him, we are a very close family, and they - we - found this very, very hard. But at the same time, I'm a soldier. I grew up in Israel, I was in the Israeli army, and I know, to be effective, you have to aim right, and you can't aim right if you're crying or angry.'

In the desperate weeks that followed, while Mariane searched for her husband in Karachi, Professor Pearl worked behind the scenes, exploring every avenue to appeal to the abductors: Muhammad Ali, Jesse Jackson and even officials from Hamas all wrote letters, at his behest, requesting Daniel's immediate release. 'I never gave up hope,' he recalls. 'I honestly believed that Danny would charm his way out. He was gifted at winning people over and I told my wife: 'He's probably playing music or soccer with his abductors.' Danny was a classical violinist and an outstanding communicator. I had complete faith in him.'

But on 22 February came devastating news. A shocking videotape released by his kidnappers showed Daniel speaking his last moving words - 'My father is Jewish, my mother is Jewish, I am Jewish' - and as he did so, a hooded man suddenly stepped forward from behind and cut his throat. 'To lose your son in such a way,' he looks down, momentarily silenced, 'it was,' he pauses again, struggling to find the words, 'very, very hard, especially for my wife who can't stop imagining what happened to him. Me, I'm a soldier, I have the power of illusion, I tell myself it took place fast, painlessly, that he was told he'd be freed after the video, that he did not know he'd be slaughtered.'

The date of Daniel's murder was put at 31 January, but it would be four months before his burnt and dismembered body was found outside Karachi. 'It re-opened the wound,' he says, 'and it was a wretched time, but I was glad to have his remains, and we buried him in a Jewish cemetery on the outskirts of Los Angeles overlooking the concert hall where he used to play with his youth orchestra.' Meanwhile, his killer - English public schoolboy Ahmed Omar Saeed Sheikh, 29, who was radicalised at an al Qaeda training camp after dropping out of the London School of Economics - was captured and sentenced to hang in Pakistan.

Professor Pearl's reaction to the video - he has only seen the first part - is acute distress, but shot through, unexpectedly, with pride: 'His statement at the end was made with total calm and dignity. How many people would have managed that?' The rest of the video he could not bring himself to watch.

In and amongst the grief and anger that engulfed him, Professor Pearl, a secular Jew, found himself acutely missing the fact that he and his son would never play music together again. But he also found himself pondering his son's poignant last words: 'My father is Jewish' I am Jewish.' 'I thought I knew what it meant for Danny,' he says, 'living life to the full with love and compassion, and treating your religion like poetry - as ripe for interpretation. I had never articulated it, but I realised that was my meaning, too.'

Within weeks, Professor Pearl had set up the Daniel Pearl Foundation, which aims to use the three vocations in which his son excelled (journalism, music and communication) to articulate his vision as a humanitarian Jew, and specifically, to help bridge the gap between Muslims and Jews. 'I realised that the real enemies are the ideologues who disseminate this culture of hate. I'm talking about the Muslim clerics who preach hate, the Arab intellectuals educated in the West who've benefited from our open society but lie that we're out to humiliate their people, and media, like Al-Jazeera, which fuels anger and hopelessness in Muslim countries.'

Professor Pearl needed a Muslim partner to realise his plan, and when he visited Professor Ahmed, he realised he had found his man. 'I saw that we have a common Utopia - democracy and freedom for the Muslim world and a two-state solution for Israel,' he says. 'He doesn't support Israel, but he doesn't advocate its destruction either, so we have disagreements on how to achieve this Utopia - which is why our public debates to search for common ground are so frank and fruitful.'

Their audiences, he says, are typically half-Jewish and a third Muslim, and when they visited the School of Oriental and African Studies in London, it was a full house. 'We have only two ground rules: nothing is taboo, and civility at all times.'

The foundation, with an annual budget of $400,000, offers two six-month fellowships a year whereby Muslim journalists from south-east Asia and the Middle East are given experience in newspapers like the Wall Street Journal and Los Angeles Times. 'The aim is to teach them the dynamics of a free press and hope they will become a voice of enlightenment.'

As he talks, Professor Pearl pulls a photograph of his grandson, Adam, now four, from his wallet. In the picture, the boy is leaning his head against his hand. 'He's a thinker, just like his dad,' he says wistfully. Adam, he reveals, is beginning to ask questions. 'He knows his father was killed by a 'bad man' - but not the circumstances.'

Has Professor Pearl ever wanted to meet his son's killer, who languishes in jail awaiting his appeal? 'I have no interest in that, 'he says. He grabs my arm. 'But I would like to meet his children - because they are the next generation.'

As we part, I tell him I would have loved to have met his son, who was my age, and whose approach to life I admired. 'Yes,' he says, putting his arm around me, his eyes filling with tears for the first time, 'he would have loved you. To lose a son is like losing your future.'

Does time heal? He shakes his head. 'The raw pain and grief are the same now as four years ago. Some people say this foundation is my attempt to replace the future. But I do it because it's effective, not because it's therapy.'

Does he really believe his mission to bridge the gap is winnable? 'Absolutely!' he booms. 'The Muslims I meet are compassionate, enlightened and willing to engage. Some people tell me: 'Oh, but you only see a select group, those Muslims who have goodwill.' I don't know if it's true, but I do believe that sanity is as contagious as madness.'