A six-year-old girl and five Hamas supporters died in an explosion near a crowded Gaza City beach last night.

Another 15 people were wounded in the blast, which went off under a car parked near a group of Hamas activists at a picnic.

In retaliation Hamas security launched a large scale crackdown on rival Fatah supporters today. Dozens have been arrested, with grenades hurled at the home of one Fatah leader, and checkpoints have been set up across Gaza.

Palestinians with the body of a Hamas fighter killed in an explosion on a Gaza City beach last night

The cause of the explosion has not been established, but if was a deliberate attack on Hamas it would be the deadliest since the Islamic militants ousted Fatah from power more than a year ago. 

Pictures of the car published on a Hamas-linked web site showed a white sedan with shattered windows and bent, damaged doors. A bearded Hamas policeman stood next to it, clutching an AK-47.

In a statement the Palestinian Health Ministry said three Hamas members and the girl were killed instantly, while two more Hamas activists died of their injuries today.

Among the dead were a senior Hamas field commander and Iyad al-Hayeh, a nephew of Hamas lawmaker Khalil al-Hayeh, they added. Osama al-Hayeh, a son of the lawmaker, was wounded.

On Gaza's streets Hamas policemen have set up checkpoints on major routes and all roads leading to security compounds, checking suspicious cars and forcing others to slow down.

Dozens of Fatah supporters were rounded up, said Ibrahim Abu Naja, a prominent Fatah leader in Gaza.

In one attempted arrest, Hamas police fired four rocket propelled grenades at the home of a local Fatah leader and briefly exchanged fire with his guards, witnesses said. Police then searched the house, but didn't find the wanted man, Mohammed Aweidat.

Mr Naja condemned the explosion and called for an independent committee to investigate.

The office of Palestinian President Mahmoud Abbas, who heads Fatah and controls the West Bank, alleged that the blast was part of score-settling within Hamas.

Hamas officials are preparing to hold an emergency government meeting later today.

Sami Abu Zuhri, a leading Hamas official in Gaza, accused Fatah of being behind the explosion.

'We hold remnants of the criminal groups and their leaders who fled to Ramallah responsible for this crime,' he said, referring to senior Fatah leaders who fled to the West Bank city of Ramallah after the Hamas Gaza takeover last year.

The beachside explosion was the third of the day.

The first went off outside the Al Jazera cafe, killing the bomber and wounding three people. The cafe, seen as a symbol of Western influence, has already been hit on two separate occasions this year.

A bomb also exploded outside the house of a Hamas lawmaker, Marwan Abu Ras, causing some damage, but no injuries. A Fatah activist was arrested.

During the past year of Hamas rule, several bombs have struck Hamas targets with Islamic militants blaming Fatah for attempting to destabilise the area. Assailants have also attacked music shops and Internet cafes, targeting them as signs of Western culture.