GAZA CITY, Gaza Strip (AP) - Hamas security forces fanned out across a tense Gaza Strip on Sunday, following a mysterious weekend car bombing that killed six people and sparked the toughest Hamas crackdown against its Fatah rivals in months.

Human rights groups said Hamas released more than a dozen of the at least 160 Fatah men it arrested Saturday in connection with the bombing, which killed five Hamas men and a 6-year-old girl. But Hamas police remained deployed in force around Gaza City, manning roadblocks and checking cars.

Fatah has denied involvement in the blast.

The Islamic Hamas and the largely secular Fatah carried on a violent rivalry for control in Gaza for years. In June 2007, Hamas routed Fatah's forces and seized control of the territory. There have been few signs of Fatah resistance since.

Violence erupted in one Gaza City neighborhood on Sunday when Hamas police exchanged fire with members of a small militia known as the Army of Islam during a raid targeting the group, a militia spokesman said. It was not immediately clear if the operation was connected with the broader crackdown, and there was no immediate confirmation of the raid from Hamas.

One militaman was killed and several people were wounded, though it was not immediately clear if they were from the militia or Hamas, a Gaza medical official said. The militia's spokesman, identifying himself only by the nom de guerre Abu Hassan al-Makdesi, said there were casualties on both sides.

The Army of Islam is not allied with Fatah, but is heavily armed and is seen by Hamas as a threat to the public stability it has largely managed to enforce since taking control of Gaza a year ago.

Gunfire was audible from a Gaza City neighborhood dominated by the group.

Islam Shahwan, a Hamas police spokesman, said his men seized "huge amounts" of explosives and weapons from Fatah in Saturday's operations. He did not elaborate.

Hamas forces "will act harshly against anyone involved in the horrible attack at the Gaza seaside, and he will be brought to justice. Anyone who is not guilty will be released," Shahwan said.

The West Bank is still controlled by President Mahmoud Abbas of Fatah, who heads a Western-backed government and is holding peace talks with Israel.

Hamas leaders blamed Fatah for Friday's explosion. The Palestinian Center for Human Rights in Gaza reported Hamas security officials arrested at least 160 Fatah loyalists, and some 40 institutions connected to the group were raided. Fatah leaders said more than 200 of its people were rounded up, including its top leader in Gaza, Ahmad Nasser.

An Abbas aide said the crackdown is reducing prospects for eventual reconciliation.

If Hamas were targeted, it would have been the deadliest such attack since the Islamic militants overran Gaza.

The blast went off underneath a car parked near a crowded Gaza City beach on Friday evening, Hamas officials said. A group of Hamas activists was picnicking on the beach at the time, Hamas said.