Arik Ascherman is sitting inside a fortified and heavily guarded Israeli police compound in the West Bank. With him are two Palestinian farmers he has persuaded to report a theft, and a uniformed officer whom he is educating in the story of Alexander the Great and the Gordian Knot.

The rabbinical homily has a message which is not lost on the policeman – much the calmer of the two officers with whom Ascherman has just had a noisy argument in Hebrew over why and whether the farmers did not report the loss of their ladders a fortnight earlier, and why he did not tell them to go to the District Co-ordination Office in Qalqilya instead of taking up the officers' valuable time here. Indeed, the policeman's enraged colleague has just stomped off after shouting at the rabbi: "You are doing an injustice to these people. I will tell you a secret. You may need them but they don't need you. If they want to make a complaint they can go to the DCO without you."

But Ascherman is unruffled. After calmly recounting how Alexander cut the Knot, he sums up the parable's point: "Philosophically, it's important for me to know what did or not happen two weeks ago; pragmatically I want to solve this problem so these people can make their complaint and we can proceed to get their property back."

In the end the farmers make their reports; a victory which has taken two and a quarter hours. "In the greater order of things, it isn't much," says the American-born rabbi with a sigh. "But it's part of the reality here."

Ascherman knows this "reality" as well as anyone. He is director of Rabbis for Human Rights, an activist organisation which he co-founded 15 years ago and which describes itself as the "rabbinic voice of conscience in Israel". If this sounds a high-flown ideal, Ascherman's pursuit of it is sometimes dangerous and always practical – at no time more so than during the annual olive harvest in the West Bank.

This sunny late-October morning, has turned out to be the day of the stolen ladders. It started while Ascherman was checking on a group of the Israeli volunteers he assembles each year to help protect olive picking Palestinian families from settlers during the harvest in olive groves overlooked by the red-roofed Jewish settlement of Kedumim. A phone call alerted him to the fact that settlers had vandalised an empty Palestinian car parked on the main road on the other side of the settlement, below the deserted – and even in Israeli law illegal – hilltop outpost of Shvut Ami.

After a 10 minute drive to the scene, he made a quick inspection of the aged white Mercedes, its windscreen smashed, its gear lever bent out of shape. Camera in hand, he strode at an athletic pace up through the scrub on the steep, rocky hill in the hope of confronting the vandals. Despite his age – a youthful-looking 50 – and his blue shirt, multi-coloured kippa, chinos and trainers, he looked like the popular image of Jesus.

Though attacked several times in the past by settlers, he seemed impervious to the potential danger or the hill's status – of which he would be fiercely reminded by police a few minutes later – as a closed military zone. The miscreants had escaped, but at the summit, Ascherman saw a ladder, the kind Palestinian farmers use to pick olives high in their trees. It was lying on the ground beside a deep stone cistern. Two others had been hurled to the bottom, where they were irretrievable without a rope and pulley.

When the police arrived, a shouted stand-off ensued, between officers ordering his group to come down, and Ascherman demanding they came up to inspect the scene of the crime. Seeing they would not do as he said, Ascherman grabbed the heavy ladder, and went down the hill with his prize at the same speed he had gone up it, as sure footed as a mountain goat.

For the rabbi already knew from his own staff that that the two farmers, Mohammed Abed, 46, and Ali Abdel Qadr, 53, had lost their ladders. In the lengthy debate that followed with police and soldiers now assembled at the foot of the hill, he persuaded the soldiers to promise they would bring an engineering unit to extract the other stolen ladders, provided the farmers made a formal complaint to the police. Hence the enervating couple of hours spent at the police compound.

All in a day's work for Ascherman, born in Erie, Pennsylvania, and motivated since his early 20s by Judaism's ancient concept of Tikkun Olam (repairing the world). As a young, Havard-educated religious Jew, he was, by his own account, "ejected" as a rabbinical student from his college on the grounds – which he quickly accepted – that he didn't "know enough about the real world". He came to Israel in 1981 to work for two years on a co-existence project in the Israeli-Arab village of Tamra before resuming his training as a reform rabbi.

As he puts it with a smile, "God's hand must have been involved" in his interrupted studies: by the early nineties, he was convinced that his greatest contribution to Tikkun Olam could be made here. Since then, he has had more than his share of beatings and arrests – including for running in front of a bulldozer demolishing homes in East Jerusalem in 2003. Married to a third-generation Israeli – herself a rabbi – and the father of two children, 7 and 11, he now runs an organisation which had grown from two people in 1995 to one with 20 full- and part-time staff.

Over falafel in a roadside Palestinian snack bar just outside Nablus, Ascherman explains that RHR's mission has three main headings: human rights education, including courses in pre-army colleges; social and economic justice in Israel, which has seen it, with other Israeli groups, win a signal victory in halting the country's draconian welfare-to-work project; and Palestinian human rights. This last includes a legal initiative which has reversed the takeover of hundreds of acres of Palestinian land by the settlements.

Through RHR, Ascherman employs four lawyers, but, he says: "If I had 10 lawyers, I could stop 90 per cent of the land takeovers in the occupied territories." He recalls a personal confrontation with a settler who had a fervent belief in the bibli

cal right to the whole land and told him "You seem to be reading from a warped Torah scroll." Ascherman replied – "sinking to his level, I guess" – that he agreed: "My Torah seems to have elements that are missing from yours. Like 'do not steal' and 'do not trespass'."

But it is for protection of the annual Palestinian olive harvest – and the cultivation of olive groves inaccessible without danger in the rest of the year – that RHR is most famous. The inspiration came in 2002, when Noaf abu Ghabia, a Palestinian deeply committed even at the peak of the intifada to co-existence and non-violence, and with whom RHR had joined in various symbolic Jewish-Arab tree plantings, appealed for help against settlers attacking harvesters in the village of Yanoun. RHR began bringing volunteers, and three years later won a crucial High Court ruling ordering the army to protect the harvest.

While it was, as he puts it, a "high maintenance victory", requiring a constant presence of the volunteers, Ascherman says that this year the army has – despite some exceptions – largely fulfilled the first two requirements of the ruling: protection of access to the land and of Palestinian farmers as they pick the olives. "There are farmers reaching olive trees they haven't been able to reach for 10 and 15 years," he says. What the army has been much less good at – so much so that RHR is close to returning to the High Court for a new order – is preventing the destruction of trees and theft of olives by the settlers.

Ascherman has a theory that the settlers' actions are a response to the nascent peace process, which they see as an "existential threat" to their way of life. He reels off a list of villages where olives have been stolen – sometimes before the harvest – or trees poisoned or cut down. Then he takes us to perhaps the saddest sight of this year's harvest, the scorched fields within sight of the notably hard-line settlement outpost of Havat Gilad.

Here, between 1,500 and 2,000 trees were burned two weeks ago by settlers – according to some witnesses, with troops looking on – as the "price" for the destruction by the army of two illegal buildings in the outpost earlier in the day. "We are going to be disciplined," he says before we drive to the burnt groves high on a ridge. "There are some pretty nasty people round here. If I say 'get back in the car,' we get back in the car."

Not for the first time, Ascherman sums up his motivation for rabbis and Israeli volunteers helping to protect the olive harvest. "It's the just and right and Jewish thing to do," he says. "But it's also the self-interested thing to do. Only I as an Israeli can empower people like Abu Ghabia to be listened to by his own people, and only people like him can empower me to be listened to by my people."

It's work which sees him leaving his home every day during the harvest for the West Bank at around 8am, not returning till around 8 or 9pm. "Sometimes I try to make it back a little earlier," he says. "My kids have human rights too."

Tree of life for Palestinians

* The Palestinian olive trade dates back thousands of years. There are around 15 million olive trees on Palestinian soil, which accounts for over 50 per cent of agricultural land in the region.

* Around 10,000 new olive trees are planted each year in the West Bank, of which about 25 per cent of the produce ends up as olive oil.

* Olives are a staple of the national cuisine, used as both food and oil. Many local growers still use traditional farming methods, refusing to use pesticides or herbicides on their trees. Palestinian olive oil has a distinctly strong, bitter and peppery flavour compared to its Mediterranean counterparts.

* The olive tree – which can live for hundreds of years – is also as a symbol of the Palestinians' determination to remain in their homeland. Over the past 10 years, it's estimated that Israel has uprooted about 1.2 million fruit-bearing trees, the majority of which were olive trees.

*Around 100,000 Palestinians are employed by the olive industry, which generates almost $100m (£62m) a year.