Forces controlled by the Hamas movement have detained more than 100 members of rival factions in Gaza.
The detentions come a day after five Hamas gunmen and a girl were killed in a bomb blast outside Gaza city.
The explosion late yesterday near a beach outside Gaza city was the deadliest incident in weeks in the territory, which has been ruled by Hamas for more than a year.
The cause of the explosion was not immediately known but Hamas blamed Palestinian president Mahmoud Abbas's Fatah movement, accusing it of collaborating with Israel.
'This criminal act proves that the call for dialogue from Ramallah (Abbas's headquarters) was a lie designed to throw sand in our eyes and conceal a conspiracy to kill and assassinate and terrorise our security forces,' Hamas said in a statement.
In the hours after the attack, Hamas-run security forces arrested dozens of people in sweeps across Gaza city, mostly Fatah members, according to witnesses.
Heavy fighting took place overnight in Gaza's Tel al-Hawa district, where Hamas gunmen battled with the family of a man they sought to arrest, exchanging gunfire and volleys of rocket-propelled grenades.
A senior Fatah official who asked not to be named said security forces arrested over 100 Fatah members and raided his party's offices across Gaza, confiscating computers and documents.
Most of Mr Abbas's loyalists were driven from Gaza in June 2007 when Hamas seized power following a week of bloody street battles between the rival factions.
'Mysterious explosion'
In a statement released by Mr Abbas's office, Fatah denied any involvement in the 'mysterious explosion' and accused elements within the armed wing of Hamas of planting the explosives as part of an internal conflict.
It was the deadliest day in Gaza since a truce was agreed between Hamas and Israel in June, which halted what had been near-daily clashes between Palestinian militants and Israeli troops.
The five men killed were all members of Hamas's armed wing the Ezzedine al-Qassam Brigades. Another 22 Palestinians were wounded in the blast, which security officials said was caused by a bomb placed outside a car.
The dead militants were identified as Iyad al-Aya, Amir Al-Sabeh, Nidal Mubeihed, Hassan al-Helo, and Osama al-Helo, a senior Gaza City commander.
A Hamas leader, Khalil al-Haya, and the Ezzedine al-Qassam Brigades accused Palestinian 'groups who collaborate with the (Israeli) enemy' of being responsible for the explosion.
'These groups are trying to undermine the security of the Palestinian people and are thereby helping the enemy,' Mr Aya told wire services, calling on Hamas officials and fighters to be careful.
The Brigades also said that several suspects had been arrested in Gaza without giving further details. Among those seized was a cameraman for German television ARD, Sawah Abu Safeh, Palestinian officials said.
Palestinian factions have clashed rarely since Hamas seized power in the impoverished territory of 1.5m people despite an Israeli blockade aimed at undermining the Islamist movement, which is pledged to Israel's destruction and considered a terror organisation in the West.
