Hamas security forces arrested dozens of supporters of the rival Fatah group early Saturday, following a late Friday mysterious explosion near a beach outside Gaza City that saw five Hamas militants and a five-year-old girl killed.

The cause of the blast remained unclear, though the circumstances suggested it was not set off inadvertently by militants mishandling explosives.

The explosion went off late Friday in a car parked near a crowded Gaza City beach. Hamas said that at the time, a group of Hamas activists was picnicking on the beach.

Three Hamas members, including a field commander, and the girl were killed immediately. Hospital officials said two more Hamas activists died of injuries Saturday. Fifteen people were wounded.

The five men killed were all members of Hamas’s armed wing the Ezzedine al-Qassam Brigades. Another 15 Palestinians were wounded in the blast.

If the blast targeted Hamas activists, it would be the deadliest such attack since the Islamic militants ousted Fatah-allied security forces from Gaza in a violent takeover more than a year ago.

One Fatah member, told The Associated Press on condition of anonymity, that at least 40 of the group’s supporters were rounded up early Saturday.

Gaza’s Hamas government did not accuse Fatah directly of involvement, but was to meet Saturday to discuss the incident.

The Hamas movement suggested the attack was politically motivated.

"Everyone should understand that political freedom is guaranteed in the Gaza Strip, but that criminal action is prohibited," a Hamas statement said.

"These crimes cannot pass. We will deter the criminals with strong punishment, to makes others think a thousand times … before they think of committing such crimes."

Hamas seized power in the impoverished coastal strip in June last year after routing Fatah-led security forces in a week of bloody street battles.
 
(Agencies via Alarabiya)