BEIT LAHIA // When they later recalled the siege by Hamas security forces, it was not its ferocity that astonished the residents of Beit Lahiya. It was the target - Shiite Muslims gathered in the building to commemorate the death of Imam Hussein, the Prophet Mohammed's grandson.

Up to 100 policemen and masked men in civilian clothes stormed an apartment building in the Gaza enclave late on Saturday.

Minutes later, they emerged dragging 15 men, whom they then beat with truncheons and denounced as infidels. Neighbourhood residents drawn to the commotion said they were shocked.

"The police showed everyone black Shiite headbands and were yelling to the crowds, 'Look at these kafirs [unbelievers]!'," said Yasser Ziada, 23. "It was like they were putting on a show for us, beating them in front of everyone. No questions - just beatings."

The onlookers had not known that Shiites lived among them.

If they occasionally referred to Hamas as "Shiites", it was because the rulers of the Gaza Strip received money from predominantly Shiite Iran.

Everyone knew, though, that members of Hamas - like every other Muslim in the Gaza Strip - were Sunni. Or so they thought.

For others in the Gaza Strip, Saturday's anti-Shiite crackdown was an epiphany for another reason.

The main allies of Hamas, Iran and the Lebanese movement Hizbollah, are predominantly Shiite, or in the case of Syria's Alawites, an offshoot of Shiism.

For years they have formed an axis of revolutionary Islam that has concerned the predominantly Sunni governments of the Middle East and their allies in the West.

Saturday's crackdown on Shiites - occurring as Hamas dismantles its headquarters in Damascus amid Syrian president Bashar Al Assad's political troubles - is an obvious affront to its long-time patron and may be a sign that one strut of that axis is rickety.

It also may be an indication that the tectonic political shifts underway since the Arab Spring erupted last year may be affecting the Gaza Strip.

"Because Hamas is straying from this Hizbollah-Syria relationship, that means they are freer to do these kinds of things," said Hani Habib, a political analyst and writer, who lives in Gaza.

That freedom, which has also seen Hamas gravitate towards newly empowered Sunni Islamist groups in the Arab Spring countries of Egypt and Tunisia, opens opportunities for hard-line Hamas members to settle sectarian scores at less cost, Mr Habib said.

In a statement released after the assault, the interior ministry in Gaza admitted carrying out the operation.

But it denied attacking Shiites, saying only that the people gathered in the building were seeking to create a "fitna", or societal crisis.

"The Gaza Strip and Palestine in general is a society that believes in Sunni Islam," said the statement, which added that there were "no Shiites in Palestine".

Some Hamas officials acknowledge, however, that Shiism has a toehold in the Gaza Strip.

Mustafa Sawaf, an official at the culture ministry and an expert on Gaza's Islamist groups, said there was a small but increasing number of Shiite converts, some of them fighters from such groups as Islamic Jihad, who had received training in Iran.

"They are growing in number, but we are a Sunni society, so Hamas felt like it had to take action against them," he said, adding that Saturday's attack was "dramatised [by Hamas] to show people in Gaza that it does not tolerate Shiites".

At the centre of the Shiite group was Mahmoud Joudeh, 52.

For years, he led a group of Salafi Muslims that lived in a compound near the Gaza city of Rafah.

Called the Excommunication and Migration Group, its members deemed greater society as un-Islamic and lived in isolation, according to a May 2010 report about Salafism in Gaza by the Institute for Middle East Studies, a research organisation in Washington DC.

Mr Joudeh recently turned away from Salafi teachings and towards Shiism, Mr Sawaf said.

At his compound on Tuesday, he received dozens of visitors wishing him a quick recovery from the wounds he said he sustained during the attack.

In an interview, he described how Hamas police violently disrupted the commemoration of Imam Hussein's death.

"They just burst into the house and started beating us," he said, displaying a cast on his right leg and left arm, both broken during the violence. "There were no questions. They just started breaking our bones until none of us could stand."

Mr Joudeh said that under the circumstances, the group's particular brand of Islam was not relevant.

"We are Muslims and no Muslim should do to a fellow Muslim what was done to us on Saturday."