The boys sitting in the shade of an awning erected on a Gaza beach are only half listening to the man addressing them through a megaphone.

After all, school's out for the summer and there is football to be played and the sea to be swum in. Some of the 100 or so boys whisper among themselves, others are busy burying their own or a friend's legs in the hot sand.

But when the man asks, "What is our slogan?" they snap to attention, responding in unison: "Resistance!"

This is summer in Gaza, Islamic Jihad-style. These boys are among 10,000 or so children that the militant organisation estimates attends its 50 camps. Hamas, the Islamic party which runs Gaza, claims another 100,000 children are attending 500 camps it organises; both are dwarfed by the 250,000 taking part in the United Nations Relief and Works Agency's Summer Games across the Gaza Strip.

Gaza's summer camps are seen by militant organisations as an opportunity to influence a generation of children; to inculcate a duty to resist the Israeli occupation of Palestinian land. UNRWA says it just wants the kids to have fun.

This year, the rivalry has taken an ugly turn with middle-of-the-night arson attacks on two UNRWA camps, one involving a death threat to the organisation's head, John Ging. UNRWA says it doesn't know who was responsible and has asked Hamas to investigate.

Down on the beach, Hasan Abdu, the Islamic Jihad official in charge of the summer camps programme, is telling the boys: "Anyone who makes concessions on Palestine is making concessions on the Qur'an. Palestine is our right. You are the men of the future – one of you might make history."

When he asks who will join the resistance in the future, hands shoot up, showing the words are getting through despite the boys' apparent inattentiveness.

The camp is named in honour of "the martyrs of the freedom flotilla", in reference to the nine Turkish activists killed by Israeli forces while trying to break the blockade of Gaza. Many of the boys are wearing T-shirts adorned with a picture of the Mavi Marmara, the flotilla's lead boat.

Zidan Obied, who is running this camp, explains the programme and philosophy. "We are expressing our principles as Islamic Jihad. We believe in the right of resistance and we are against peace negotiations."

He runs through some of the daily activities: sessions on the history and geography of Palestine; readings from the Qur'an; arts and literature; drawing – "we teach them to draw maps of Palestine from the river to the sea"; lessons on the significance of Jerusalem and the al-Aqsa mosque; sports; volunteering activities such as tree-planting and clearing cemeteries; military-style marching and exercises.

This, of course, is for the boys. There are separate camps for girls, with "very limited" sporting activities. Instead they are taught crafts, such as embroidery.

"We are a conservative community, we try to avoid bad behaviour between boys and girls," says Obied. "Dealing with girls is different."

Many families are reluctant to allow their daughters to attend even segregated camps, whoever is running them, and across the board there is a disproportionate number of boys taking part in organised summer activities.

A few miles to the north of the Islamic Jihad beach awning, separate Hamas-run boys' and girls' camps are sharing the same seaside facility. The girls here are studying the Qur'an for five hours a day; the boys have a wide range of activities on offer, including football, computer skills, marching and a "sniper's corner" where they are taught shooting as a sport.

The children are also taught about the history of the Palestinian struggle. In a society where politics and conflict is part of daily life, the notion that children should be allowed to enjoy childhood free from such burdens is incomprehensible.

"Of course we have a political agenda," says Ahmed Nabil, a Hamas official helping to run the camp. "We believe the older generation has a duty to tell the younger generation about these issues. We are letting them play but also giving them a message. We must not let them forget that we are an occupied people."

Israel claims the Hamas and Islamic Jihad summer camps are breeding a new generation of extremists. UNRWA declines to comment on other camps, but emphasises its own, contrasting, philosophy.

"In the highly charged, pressurised environment of Gaza, it's important to have a space where children can just be children," says UNRWA spokesman Chris Gunness. "We want to give the children a sense of fun and normality."

UNRWA's camps are better organised and equipped, not surprisingly given the organisation's resources. Under brightly coloured streamers at camp number nine, there is a high-sided portable swimming pool, bouncy castle, trampoline and volleyball net.

The girls come in the morning, the boys in the afternoon – but both follow the same programme of activities, which sometimes involve a theme of teamwork and mutual respect.

Mohammed Zyara, running the boys' activities at the camp, says: "Our main goal is to give them a good time, keep them away from troubles and politics."

However, he says, UNRWA does not discourage children from attending rival camps. "I'm sure some of the boys go to one kind of camp in the morning, and another in the afternoon," he says philosophically.

Back at the Islamic Jihad camp, the political lesson is over and the boys are playing in the waves despite the 80m litres of raw or partially treated sewage discharged into the sea off Gaza every day. Under the watchful eye of camp organisers, they chorus their support for Islamic Jihad, although many have also attended UNRWA camps this summer.

Hasan Sidan, a 13-year-old whose hair and clothes are caked in sand, reassuringly reflects the priorities of most boys his age. He likes the high jump best, and "the worst thing is when they are lecturing us". Most of all, he says, he just wants to play on the beach and have a good time.