They preach global jihad, or holy war, adhere to an ultraconservative form of Islam and are becoming a headache even for Hamas, the Islamic militant group that rules Gaza.
Jihadi Salafis, as they are known, have organized into small, shadowy armed groups that have clashed with Hamas forces and fired rockets at Israel in defiance of Hamas' informal truce.
Perhaps even more worrisome for Hamas, they claim a growing appeal among Gazans in the territory's pressure cooker of isolation and poverty, raising fears they could serve as a bridgehead for their ideological twin, al-Qaida, from which they take their call for global holy war.
Hamas insists it dismantled the groups after a mosque shootout last summer that left 26 dead.
But after months of lying low, Jihadi Salafis become active again. Besides resuming rocket fire on Israel in recent weeks, they blew up the car of a Hamas chief outside his southern Gaza home. The chief, who was not in the car, was unhurt, and the group that claimed responsibility said the blast was a warning.
"We will not stop targeting the figures of this perverted, crooked government (Hamas), breaking their bones and cleansing the pure land of the Gaza Strip of these abominations," said the group, the Soldiers of the Monotheism Brigades. "What will come next will be harder and more horrible."
Going by names like "Rolling Thunder" and "Army of God," they oppose Hamas for refraining from imposing Islamic law since seizing power in Gaza in 2007 and largely sticking to a tactical truce with Israel since the latter's devastating offensive last year.
Expert opinion holds that al-Qaida has shown little interest in inviting the Gaza groups it inspired into the fold. But even an al-Qaida foothold in Gaza could pose a significant challenge to Hamas' control as well as its attempts to get off Western governments' terrorist list and lift the Israeli-Egyptian blockade of Gaza.
Hamas own rapid rise to power is a reminder of the appeal of militant ideas in the absence of a peace process.
Gaza's Hamas prime minister, Ismail Haniyeh, acknowledges that some in Gaza have been swept up by the ideas of the Jihadi Salafi groups.
"If this is a phenomenon among some young men in Gaza, they will be treated with discussions and meetings," said Haniyeh in a sermon to mosque worshippers. However, he rejected any suggestion of an al-Qaida presence in Gaza and repudiated the call to global jihad.
Still, Hamas may inadvertently have helped create a climate for Salafi growth with its own gradual push to make Gaza more Islamic, including a "virtue campaign" that urges women to cover up. But Hamas has stopped short of a direct Taliban-style assault on secularism.
"It is more difficult for Hamas to deal with these people because they are selling the same goods: religion," said Mahmoud Abu Rahmeh, a Gaza human rights researcher.
The Salafi movement has grown across the Middle East, preaching an ultraconservative Islam similar to Saudi Arabia's, strictly segregating the sexes and interpreting religious texts literally.
Salafis tend to be nonpolitical, but a minority jihadist stream embraces the al-Qaida call for holy war against the West and the moderate Arab leaders in its camp.
Hamas, on the other hand, confines itself to pushing for a Palestinian state, says the sole target of its suicide bombings and missile attacks is Israel, and makes compromises with other movements, even participating in Palestinian elections in 2006.
Those stances are reviled as un-Islamic by the Salafi purists.
Their groups began to emerge in Gaza after Israel withdrew from Gaza in 2005. A study co-authored by a former deputy chief of Israel's Shin Bet security service estimates their membership in the low hundreds, including disgruntled followers of established Palestinian militant groups.
The Jihadi Salafis are suspected in a series of bombings of Internet cafes and music stores in Gaza, seen as purveyors of vice.
In June, a group called Jund Ansar Allah (Soldiers of the Supporters of God) sent explosives-laden horses toward an Israeli border post, but the attack was foiled and four fighters were killed in a battle with the Israelis.
Hamas praised the four dead as martyrs, but then faced its most brazen challenge when Jund Ansar Allah's leader, Abdel Latif Moussa, flanked by masked gunmen, took to the pulpit of a mosque in August to proclaim Gaza an Islamic emirate.
Hamas raided the mosque and 16 Salafis, including their leader, died in a gun battle, along with five Hamas men and five civilians.
Hamas arrested about 150 Salafis across Gaza, including former Hamas members. Twenty-five remain in prison, said Interior Ministry spokesman Ehab Ghussein.
One of those released spoke to The Associated Press in his family home in the southern town of Rafah. The 21-year-old college student insisted on anonymity, saying he feared Hamas retaliation.
He said he was in the mosque during the shootout, though unarmed. During nearly two months in a Hamas prison, he said, he was beaten and Hamas men cut off his long hair.
He said he had joined Hamas as a teen, but left when the movement participated in Palestinian elections in 2006. Democracy is wrong, the young man explained, since rule should only be by Islamic law.
"I felt Hamas was making too many compromises," he said.
The student wore a white prayer cap and shin-length robe, a style Salafis believe emulates the dress worn at the time of Prophet Muhammad.
The violent Salafi groups are inspired by al-Qaida but are not formally affiliated with it, according to a January study by the Washington Institute for Near East Policy, a U.S. think tank, co-authored by Yoram Cohen, the former Shin Bet official.
It said Al-Qaida has not established an affiliate in the region nor accepted any of its "locally radicalized, globally inclined jihadists."
The U.S. think tank, citing Israeli officials, estimated that 30 to 50 fighters from Yemen, Egypt, France and elsewhere have slipped into Gaza, either to train Salafi fighters or to wage holy war. But the authors said none are believed to be al-Qaida operatives.
The group called Rolling Thunder, which pledged allegiance to bin Laden in a July statement, claimed that some of its fighters went abroad for training.
Hamas denies any foreign fighters are in Gaza. However, in an apparent sign of concern, its radio is broadcasting warnings to owners of Gaza's blockade-dodging tunnels not to let foreigners through.
The Shin Bet and Israel's military intelligence did not respond to requests for comment.
Beyond the armed groups, the nonviolent Salafi movement is taking root in Gaza. Its adherents also believe that jihad is a religious duty, but put more emphasis on returning to what they consider the real Islam in their daily lives.
Salafis run charities that include Quran lessons and feeding the poor.
On a recent day, a dozen women, more heavily robed and veiled than most Gaza women, studied Quran in the Rafah office of the Ibn Baz charity.
The message is strictly religious, said Sheikh Hussam al-Gazar, the charity's deputy director.
"People are returning to the real Islam, after seeing that they get no benefit from political parties," he said.
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