A powerful explosion ripped through a car on a busy Gaza City beach Friday night, killed three and injuring 20, Hamas security officials said.

It was the third mysterious blast of the day in Gaza after a relatively calm period that has followed a cease-fire between Israel and the territory's Islamic Hamas rulers. No one in Gaza blamed Israel for Friday's violence and it is likely internal Palestinian battles.

The late night blast killed Amar Musubah, a Hamas military field commander, Hamas militant Eyad Al-Hia, a child and a fourth unknown individual. All the wounded were civilians, medical officials said.

Earlier in the day, unknown assailants set off two bombs in Gaza City, killing one man. The first explosion took place just after midnight outside the Al Jazera cafe. The cafe has been hit twice in the past year by similar attacks, presumably perpetrated by radical Muslims who also target record shops and institutions they see as signs of Western influence.

The powerful blast wounded three people and killed the man who tried to detonate the device, Hamas officials said.

The second bomb exploded outside the house of a Hamas lawmaker, Marwan Abu Rass, not long afterward, causing light damage but no casualties, they said. A Fatah activist was arrested as a suspect in the incident.

Hamas officials said police were investigating all the blasts.

Gaza is the scene of regular bloodshed between Israeli forces and Palestinian militants, though the territory has been quiet over the past month because of a cease-fire between Israel and the territory's Islamic Hamas rulers.

Gaza is also a common sight of internal Palestinian violence between the Hamas rulers and their Fatah rivals. Hamas forces violently overthrew Fatah a year ago, and tensions remain high.

Violence has also been blamed on fundamentalists who oppose what they see as signs of Western cultural encroachment.

In recent years, shadowy groups have firebombed internet cafes, music stores and Christian institutions. The bombings are typically carried out late at night and most have caused no casualties. But in one deadly attack in October, 2007, a local Christian activist was murdered. No arrests have been made in the case.