On a humid afternoon, an hour or two after lunch, Nadi al-Attar, 12, set off on a donkey-drawn cart with his grandmother Khariya and two of his young cousins to pick figs from a small orchard near their home in northern Gaza.

Ahmed, 17, one of the cousins, remembers the moment when the shell struck, but pauses as he tells his story to nervously rub the muscles at the top of his thighs. The shell that hit their cart that afternoon sliced off his left leg just above the knee and his right leg halfway up his calf. He still has an aching pain in his bandaged stumps.

They had stopped the cart and two of the boys jumped off. "They went to collect something, some metal bars, and then they came back to the cart," he said. The boys hoped to sell the strips of metal for scrap. The Palestinian Centre for Human Rights (PCHR) later determined that the metal came from a launcher for a Qassam, one of the crude rockets launched by Palestinian militants from Gaza into Israel. Qassams had been fired from the area that morning, though the militants had since left.

"Then the shell struck. I saw my mother [Khariya] dead and Nadi killed. I saw them dead on the ground," Ahmed said. "I looked down and then I saw my legs were cut away."

Human rights field workers believe an artillery shell, fired from an Israeli military position not far away at the border with the Gaza Strip, hit the cart. Several were fired that day, July 24 - one day in a long and damaging Israeli military operation.

"I think it happened because of the metal we were collecting," Ahmed said. "But we were just going to the farm." He was taken to hospital with another cousin, Shadi, who was wounded in the stomach by shrapnel. Nadi and Khariya, 58, were killed instantly.

"We had lunch together," said Nadi's father, Habib, 36. "Then he went with his grandmother and never came back."

The deaths are not an isolated case. For the past two months, while the world's attention in the Middle East has been focused on the conflict in Lebanon, the Israeli military has led a wave of intense operations along the length of the Gaza Strip. It began after the capture of an Israeli soldier, Corporal Gilad Shalit, by Palestinian militants on June 25. The Israeli military said its operations were intended to free Cpl Shalit and to halt Qassam rocket fire. Early on the Israelis bombed Gaza's only power plant and they have kept Gaza's crossing points to Israel and Egypt closed for most of the time.

Since the start of the operation, codenamed Summer Rain, at least 240 Palestinians have been killed. One in five were children. According to the PCHR, which has investigated each case, 197 of the dead were civilians and the vast majority were killed in Gaza. Among them were 12 women and 48 children.

Yesterday an Israeli military spokesman said his forces did not target civilians. "Our actions are targeted only at terrorist organisations, terror activities and infrastructure," he said. "It can happen that innocent people are hit. But the responsibility does not lie with the Israeli army, but rather with the terror groups who are working within civilian populations without any regard to the danger they are causing."

More than two months into the Gaza operation Israel has still not secured the release of Cpl Shalit or stopped Qassam rocket fire.

"We believe that the whole offensive against the Gaza Strip is characterised by being an act of revenge and retaliation in which civilians are paying the price," said Hamdi Shaqqura, a founder member of the PCHR in Gaza City. "They have demonstrated total disregard for the rights of innocent Palestinian civilians. There has been an excessive use of force, a disproportionate use of force in civilian areas, and that explains the high toll of death."

Mr Shaqqura also condemned the Palestinian militants for launching the Qassams and for firing them from civilian areas. "This is illegal and we have called on them to stop," he said.

Many relatives of those killed by the Israelis in Gaza have been equally critical of the rocket attacks. "We get nothing out of it," said Muhammad al-Attar, 23, another of Nadi's cousins. "After they launch rockets we get killed and they destroy our farms."

A few hours after the donkey cart was hit a shell was fired into Beit Hanoun, another district of northern Gaza. It killed Khitam Tayeh, 11, who was on her way to the shops after school with her sister Nuha, 12. Nuha was hit by a piece of shrapnel in her left thigh, but survived. Khitam had a severe head injury and died in hospital.

"I carried her in from the ambulance and took her to the operating room in my arms," said her father, Muhammad 48. "Then she died. They couldn't do anything." He showed several framed photographs of his daughter, with long dark hair and wide brown eyes. Two bright stars had been superimposed in the background.

Mr Tayeh has collected a box of shrapnel from the scene, a couple of dozen sharp, rigid shards of metal, each three or four inches long, and talks of bringing a legal case against the Israeli military. Like many, a year ago he had hoped that life in Gaza would improve when Israeli settlers were withdrawn, in what seemed a ground-breaking move.

"People expected it would get better, but it's been the opposite," he said. "Don't tell me they withdrew. It's like they didn't leave. They are everywhere."

On the eastern side of Gaza, in Shujaiya, Hussam al-Sirsawi, 12, was with his friends standing on the street watching Israeli troops fighting against militants in the distance on August 27. He was badly injured by a piece of shrapnel and died three days later.

"You know how children are when they hear something happen. They want to go and see," said his uncle Nasser al-Sirsawi, 37. "I can't say why the Israelis killed him. These army people are full of hatred. Maybe these kids went to watch some resistance people and they were in the wrong place. To kill a child like this is not natural." On the wall opposite his cloth shop there is graffiti dedicated to his nephew. "Hussam," it says, "we swear to God you won." "Of course," said his uncle, "he's a martyr."

Two days later there was another incident in Shujaiya, when again a group of children were watching the fighting. Either a tank shell or a large chunk of shrapnel flew at them and hit Muhammad al-Ziq, 14, on the head. He died instantly. "I think sometimes they just want the Palestinians to pay," said his uncle, Ziad al-Ziq, 36. "He was with children wanting to see what was happening. There was no excuse for what happened."

All of the dead and most of the injured pass through the Shifa hospital in Gaza City. Staff photograph the bodies of the dead - they call the victims "martyrs" - and document their injuries. Juma'a al-Saqqa called up a picture on his computer screen of Muhammad al-Ziq, an appalling image of the boy lying on his side on a metal morgue table, the side of his head sliced away. In the past two months the hospital's doctors have dealt with 1,280 injured from the military operations, a third of whom were children. The doctors performed 60 amputations.

Dr Saqqa flicks through the photographic record, images of bodies charred beyond recognition, flesh no longer human in form. Many of the figures were young children, at least one in a shredded blue school uniform. "We have passed through the worst situation we have ever come across in our years of work," he said. "But this is our situation. What can we do? We raised our voices to the world, but nobody moves."