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 over a dozen of the at least 160 Fatah men it arrested Saturday in connection with the bombing a day earlier, 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. But Hamas said Sunday that Palestinian security forces loyal to moderate Palestinian President Mahmoud Abbas have arrested 36 Hamas activists across the West Bank in retaliation for the arrests of the Fatah members.

After nightfall, Hamas released Ahmed Nasser, a Fatah leader, because of his health problems, family members said.

In Cairo, Abbas called for renewed talks between Fatah and Hamas.

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.

Fatah's Abbas controls the West Bank and is holding peace talks with Israel.

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.

One militiaman 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 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 leaders blamed Fatah for Friday's explosion. The Palestinian Center for Human Rights in Gaza reported Hamas 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.

Despite the arrests in the West Bank and Gaza, Abbas announced from Cairo that Egypt would begin coordinating Hamas-Fatah reconciliation talks. But a Hamas spokesman said his group had not received any formal invitation to dialogue and accused Abbas of trying to divert attention from the bombing.

Egypt frequently acts as a mediator between rival Palestinian groups.