A secretive Hamas campaign to catch Palestinians spying for Israel has ensnared some prominent Gaza residents, drawn unusual criticism and highlighted the Islamic militant group's deep fears about being penetrated by agents of the Jewish state.

Though action against accused collaborators is always popular in Gaza and tensions are hardly new in the seaside strip — a crowded and impoverished place that endures a three-year blockade that has kept key supplies scarce and made travel out for most people virtually impossible — this time seems different.

There is widespread shock at some of the well-respected names among those thought to be detained — including two prominent physicians and a respected engineer, alongside members of Hamas itself.

And mostly, there is concern about the extreme secrecy surrounding the arrests. Hamas refuses to say who has been arrested, a policy that has sparked a furious rumor mill since the arrests began earlier in September.

"Everybody in Gaza is under suspicion," said Mukheimar Abu Sada, a Gaza-based political scientist widely known for his independence, describing an atmosphere of fear in Gaza, where collaborators are widely loathed and the preferred method of their punishment is death — either through the courts or vigilantes.

Hamas feels "the government has been completely infiltrated, that Israel knows more about Hamas than what they know of themselves," Abu Sada said.

The Iranian-backed militant group seized control of Gaza from the rival Palestinian Fatah movement in June 2007. Hamas' initial crackdowns were political, targeting Fatah supporters but eventually the net widened, absorbing lawless tribes, human rights groups and extremist Muslims opposed to Hamas' rule.

Along with the crackdowns, Hamas has steadily imposed its strict Muslim lifestyle on traditionally conservative Gazans — banning women from smoking water-pipes, warning cafes not to allow men and women to mix in public, and pressuring women to wear the Muslim headscarf.

Human rights workers who are in frequent touch with security officials estimate that more than 20 low-level Hamas operatives have also been rounded up as suspected collaborators in the September arrests. Detainees have been denied access to lawyers or family visits.

A senior Hamas figure based in Lebanon, Osama Hamdan, said the group detained a number of collaborators and that some of them had confessed.

But Hamas officials in Gaza are silent on the detentions — fearing any chatter might help Israel figure out who has been arrested. Officials have called for calm and promised a more detailed public accounting soon.

"We are talking about a very sensitive issue and we will not be rushed," said Hamas interior ministry spokesman Ehab Ghussain.

The campaign is taking a heavy toll on the population.

The children of one man known to have been arrested are avoided on their school playground, residents said.

Another man taken for questioning over a deadly car accident was immediately branded as a collaborator by gossiping neighbors — a potentially lethal turn of events. The same happened to about 10 men in the southern town of Rafah who were briefly detained, most likely over a dispute with a rival family.

"Rumors ... have touched people and families and organizations that are respected in Gaza, and this has led to confusion and the shredding of our social fabric," prominent Gaza writer Mustafa Sawaf wrote in the pro-Hamas daily Felesteen, in rare public criticism of the militant group.

He said Hamas needs to explain its actions: "We need to put the complete facts before the people, even if it is bitter."

Israel has long run networks of informers in the Palestinian areas, but a game-changer for Hamas appeared to be the Israeli assassination of Interior Minister Said Siyam, killed in an airstrike during Israel's three-week assault of Gaza almost two years ago.

Siyam was one of Hamas' most senior figures and was meant to be deep in hiding. The strike raised questions who around the minister may have revealed his location, Abu Sada said.

There are also fears over what Palestinian rights activists describe as efforts by Israeli intelligence services to squeeze information out of Gaza residents who needed to enter the Jewish state for urgent medical treatment.

The September arrests followed a widely advertised campaign entitled "Repentance for Collaborators," which began several months ago with radio appeals for spies to surrender in exchange for amnesty.

Gaza Palestinians widely approved of Hamas executing three convicted collaborators in May. Another 11 people have been sentenced to death. Rights groups say gunmen loyal to the group killed 17 prisoners, mostly suspected collaborators, during the chaos surrounding Israel's Gaza offensive in the winter of 2008-2009, launched to stop years of rocket fire from the strip.

"There is no doubt that the circle is closing (on collaborators) as a result of the efforts that are being exerted," Hamdan said.