On the streets of Rafah in the southern Gaza Strip, clusters of men wear long tunics over baggy trousers, a costume common in Pakistan but virtually unknown among Palestinians — until recently.

It is an emblem of Salafism, a branch of Islam that advocates restoring a Muslim empire across the Middle East and into Spain. Some Salafis preach violence, even killing Muslims deemed not pious enough. While historically a fringe group in the southeastern Mediterranean, Salafis have sought inroads in Lebanon and Jordan and are battling Hamas in Gaza.

While Al Qaeda, which shares its conservative religious views and promotion of holy war, has not gained a foothold in the region, Salafism may be the wave of the future. In Algeria and Morocco, similar movements have expanded in the past two decades to create havoc through civilian bombings and attacks on the police.

“This is the challenge we face in the world,” said Bilal Saab, a researcher in Middle East security at the University of Maryland in College Park. “We are getting better at dealing with insurgencies, though Afghanistan is proving to be an exception. It is much more difficult to combat the constant threat of underground urban terrorism.”

Armed Salafis are challenging the authority of Hamas, the Islamic party that rules the Gaza Strip and has fought Israel for two decades. Gaza Salafis say Hamas surrendered its credentials as an Islamic resistance group when it declared a unilateral cease-fire after a 22-day war with Israel that ended Jan. 18. Hamas’s Health Ministry said 1,450 Palestinians were killed in the conflict. The Israeli Army put the toll at 1,166 Palestinians and 13 Israelis.

“They believe Hamas has been neutralized and has given up the fight,” said Mkhaimar Abusada, a professor at Al-Azhar University in Gaza.

Hamas, which is on the U.S. State Department list of terrorist organizations, is holding dozens of Salafis in jail, trying to persuade them to end their opposition, said a Hamas police spokesman, Rafik Abu Hani. “They want to implement their own ideas through weapons, and we can’t allow that.”

Arrests began after an Aug. 14 Hamas raid on a mosque in Rafah, where armed Salafis belonging to a group called Warriors of God had gathered. The group’s leader, Abdel-Latif Musa, had proclaimed an Islamic emirate in Gaza, directly challenging Hamas rule, according to a transcript published by the Middle East Media Research Institute, a Washington-based translation and analysis organization. Mr. Musa and 21 other people, including six civilians, died in the raid.

“The emirate idea was crossing a line of Hamas tolerance,” Mr. Abusada said. “Hamas basically said, ‘Don’t mess with us.”’ Since the crackdown, Salafis have been responsible for two bombings that did not cause any casualties, he added.

The Warriors of God group is among at least four armed Salafi organizations in Gaza, along with the Army of Islam, Victory of Islam and Lions Den of Supporters, Mr. Abu Hani said. Members total no more than 400 to 500, he estimated. Mr. Abusada said there were many more: between 4,000 and 5,000, including defectors from Hamas.

The next step would be for these groups to unify and organize, attract more newcomers dissatisfied with Hamas and try to forge ties with Al Qaeda, said Samir Ghattas, a Palestinian analyst at the Maqdis Center for Political Studies in Gaza, at a Sept. 30 terrorism conference.

In 2007, a Salafi group in Lebanon called Fatah al Islam held off the country’s army for three and a half months at the Nahr al Bared Palestinian refugee camp. The combat left about 400 militants and 168 soldiers dead, according to Lebanese press reports.

Remnants of the Salafi group probably have taken refuge in other Palestinian camps in Lebanon, Mr. Saab wrote in the September issue of CTC Sentinel, a publication of the Combating Terrorism Center at the United States Military Academy at West Point, New York. Also, security forces have foiled Salafi attacks in Jordan, he wrote.

The Salafist Group for Preaching and Combat has spearheaded several years of civil war in Algeria. After pledging allegiance to Osama bin Laden in 2006, the group changed its name to Al Qaeda in the Islamic Maghreb. It has bedeviled Algeria with bombings and ambushed security forces, even though membership is only in the hundreds, according to U.S. State Department statistics.

While there is no indication of any direct relationship between militant Salafis and Al Qaeda, they have become a reference point for radical groups from Morocco to Central Asia. One Salafi in the Gaza Strip town of Khan Younis, who called himself Abu Iyad, said he did not belong to any armed organizations but understood people who did.

Hamas adherents “say they resist Israel, but they stopped fighting,” he said. “Why did all the people die? Hamas is acting just like Fatah,” the movement led by President Mahmoud Abbas of the Palestinian Authority, who favors peace talks with Israel.

“What the Salafis don’t understand is we need to give the people a break; we need to rebuild and prepare for the next battle,” said Younis Astal, a Hamas member of the disbanded Palestinian Parliament. “We can’t have perpetual war. That would be inhuman. Anyway, they want to make Gaza like an Al Qaeda base, and we don’t want that.”