Palestinian gunmen killed three young children of a senior Palestinian intelligence officer Monday, pumping dozens of bullets into their car as it passed through a street crowded with schoolchildren -- an unprecedented attack that could ignite widespread factional fighting.

There was no immediate claim of responsibility, and the ruling Islamic militant Hamas denounced the drive-by attack. However, senior officials in the intelligence service, which is allied with Palestinian President Mahmoud Abbas, blamed Hamas for the shooting.

The children's father, Baha Balousheh, is considered a leading enemy of Hamas. Balousheh, a loyalist in Abbas' Fatahmovement, was a lead interrogator during a crackdown on Hamas a decade ago.

"This is an ugly and inhuman crime perpetrated by a bunch of lowlifes," Abbas said at his West Bank office in Ramallah. "We condemn it vehemently."

In Monday's attack, the children's car was riddled with some 60 bullets by gunmen driving in two vehicles, Palestinian security officials said. Its seats, school bags and a small plastic bag with a sandwich in it were splattered with blood.

Three of Balousheh's children, ranging in age from six to 10, were killed, in addition to an adult, hospital officials said. Four more people were wounded in the attack in Palestine Street, lined with nine schools. Balousheh was not in the area.

The shooting sent school kids in the area running for cover, some dropping to the ground, others fleeing in panic. "We saw fire coming from one car. We started screaming and children started running," said 12-year-old Fadwa Nablus, who had been walking to school with her 9-year-old brother.

Hours after the attack, black smoke covered the area of the schools, as young Fatah activists burned tires in protests.

At midday, thousands of people marched through the streets of Gaza City in a funeral procession for the children. Hundreds of pro-Fatah security officers were among the marchers, many of them firing into the air.

"I have no words. Words stop at the extent of this crime," the children's father said. "I am a father who has lost his children ... This crime is a part of the terrorism which continues on Palestinian streets."

Security officials said they believed the shooting was an attempt to assassinate Balousheh, noting that the car's windows had been covered by black shades, blocking a view of the passengers. Gunmen had twice before tried to kill Balousheh.

"Palestinian security has opened an investigation into the incident, which we consider the ugliest" in the growing chaos in Gaza, the officials said in a statement.

The Hamas-controlled Interior Ministry, which is responsible for security, promised a speedy investigation.

However, a senior intelligence officer in Gaza, speaking on condition of anonymity, said he believed Hamas was behind the attack. In a statement, the intelligence service stopped just short of explicitly accusing Hamas, saying the shooting was the latest in a series of attacks on intelligence officers the service has blamed on the Islamic group and its allies.

"We have information that Hamas is heading toward a new internal crisis ahead of early elections, that Hamas is responsible for this attack and I can't imagine this will go without a response from Fatah," said Jamal Tirawi, a former intelligence officer and cousin of the current intelligence chief, Tawfiq Tirawi.

Hamas spokesman Fawzi Barhoum condemned the attack as an "awful, ugly crime against innocent children." He said the assailants were undermining Palestinian interests by creating chaos and confusion.

The shooting came a day after gunmen opened fire on the motorcade of Interior Minister Said Siyam of Hamas. Siyam's car was not hit and there were no injuries.

An Abbas aide, Palestinian negotiator Saeb Erekat, said he was concerned the attack will lead to a large-scale confrontation. "If this continues, it will lead to our worst nightmare, internal fighting," Erekat said.

Gaza has been plagued by factional violence in the past, with dozens killed and hundreds wounded. However, Monday's attack targeting children was unprecedented, and was likely to trigger widespread confrontations at a time when the lines between Hamas and Fatah have hardened.

Earlier this month, Abbas announced that talks on forming a unity government between Hamas and Fatah have broken down. Earlier this week, he raised the possibility of calling early elections, drawing angry protests by Hamas which said he does not have the authority to dissolve the Hamas-dominated parliament.