A student demonstration in support of the Sharek Youth Forum in Gaza City was brutally broken up by Hamas police yesterday, following the forced closure of the group's offices last Tuesday. Demonstration organisers claim a girl of 18 was beaten and 20 others were arrested on charges of protesting without a permit. At least three are still being held.

Sufian Mshasha, co-founder of Sharek, told us he was "happy that people in Gaza were still willing to stand up for causes they believe in", but expressed fears that the forum's supporters could face further intimidation.

Sharek's liberal agenda had resulted in frequent clashes with the Hamas government prior to its closure, which has announced that the forum is now under criminal investigation on unspecified charges. Sharek staff protest the closure is illegal and unjust.

In the past seven months, the group's offices have been repeatedly raided and members of staff have been subjected to physical intimidation, harassment and threats. During this time, the xecutive manager, Muheib Shaath, has been summoned to 15 separate interrogations from internal security. A summer camp run by Sharek in partnership with the United Nations Relief and Works Agency (UNRWA) was destroyed in May.

Mshasha told us the harassment and ultimate closure were "prompted by our agenda of democracy, social development, and our insistence on holding activities for both genders". He claimed that "80-90 per cent" of questioning of Sharek staff focused on their practice of encouraging both sexes to take part in their programmes.

Mshasha believes the closure is in violation of the 2000 charitable societies and NGOs law, which states: "The closure of any society or organisation should be according to a decision issued by a court of law." Despite verbal threats and an order from the attorney general, Mohammed Abed, no legal process took place to justify the police's actions.

Sharek, which also has offices in Ramallah, has a broad mandate to promote youth empowerment in the Palestinian territories. It serves 65,000 children in Gaza, through capacity-building, education workshops and social activities. Some of these have been perceived to violate sharia law, including concerts and a mixed-gender trip to the beach.

The forum has also come under fire for its links to UNRWA, from which it receives funding, and other international organisations.

Shasha claims the group is sensitive to Gaza's conservative environment. "Our director is an observant Muslim, our IT technician wears a burqa. Almost all the women wear traditional Islamic dress and all our volunteers are from Gaza." He also denies Sharek is opposed to Hamas: "We hold all political groups accountable [for failing to promote youth empowerment], but Hamas take our actions as accusations."

Sharek has enjoyed a good relationship with high-ranking members of the Hamas administration. Prime Minister Ismael Haniyeh is said to have supported its cause, but to have been unable to protect it from more conservative elements of his government.

The Gaza journalist Mohammed Mohanna believes this exemplifies a worrying lack of central control. "There are three parts of Hamas: military, government and the mosque groups [dowas]. The mosque leaders are very powerful. They influence the government by saying, 'Look what these associations do, they are all bitches and motherfuckers, they are against Islam.'

"It creates a lot of pressure. They have been campaigning against shisha pipes in coffee shops and women without hijabs."

He believes Hamas officials infiltrate associations like Sharek in order to control them. "They change the faces, run it differently and do what they want."

The increased pressure on development groups such as Sharek was demonstrated by the recent closure of two associated organisations, although these were run by the municipality itself. "It's crazy, but sometimes the decisions are just stupid, they don't have a plan," Shasha says.

Since the closure is technically temporary rather than permanent, Sharek is unable to bring a legal case against it. The group hopes to win enough popular support, which Shasha claims has been coming in waves in Gaza, to force the government to reverse the action. Sharek is petitioning prominent politicians in the West Bank and Gaza to come to its aid.

Shasha believes the principle is too important to let go. "The fundamentalist elements measure adherence to Islam by how boys and girls dress," he says. "This latest move is a very dangerous indication of their influence."

When Hamas was elected in 2006, it was with a commitment not to impose sharia law and a pledge to accept pluralism in society. "This is why they won the election," Shasha says. "All our efforts now are to make them respect the promise."

Kieron Monks is a freelance reporter and editor of the Palestine Monitor news website, based in Ramallah.