The killing of three sons of a top intelligence officer in Gaza has caused widespread outrage among Palestinians.
Palestinian Authority President Mahmoud Abbas said the attack by gunmen in Gaza City was "an ugly and inhuman crime".
"Words stop at the extent of this crime," said the children's father, Baha Balusheh, who is linked to Mr Abbas's Fatah party.
The gunmen fired dozens of bullets at the car in which the children, aged six to 10, were travelling to school.
An adult was also killed in the attack which took place in a street crowded with children.
So far, no-one has admitted carrying out the drive-by shooting.
The motive also remains unclear but Mr Balousheh's position means he would have made many enemies, the BBC's Alan Johnston in Gaza says.
Mr Balousheh led a crackdown on the now-ruling Hamas movement 10 years ago.
"I am a father who has lost his children... This crime is a part of the terrorism which continues on Palestinian streets," said Mr Balousheh who was not travelling in the car at the time.
Hamas spokesman Fawzi Barhoum condemned the killings as an "awful, ugly crime against innocent children".
The attack comes amid growing tension between the rival Palestinian factions, Hamas and Fatah.
It is bound to inflame the situation, the BBC's Middle East analyst Roger Hardy says.
The clear danger is that political differences will lead to an escalation of tit-for-tat violence, he says.
On Sunday, gunmen shot at Interior Minister Saeed Seyam's convoy in Gaza. Mr Seyam, who is a senior Hamas leader, was unharmed in that incident.
The attack happened as children were arriving at nine schools which line Palestine Street in Gaza City's central Rimal district.
The gunmen fired more than 70 bullets at the vehicle in which Mr Balousheh's children were travelling. At least two other children were hurt.
Inside the vehicle with its blacked out passenger windows, the seats and a school bag were covered in blood.
There was pandemonium as hundreds of people ran for cover from the gunfire, with many young children being separated from parents and siblings.
Fadwa Nabulsi, 12, said she was outside a school with her nine-year-old brother, Wael, when the shooting started.
"We saw fire coming from one car. We started screaming and children started running. I was crying, and I lost Wael for about half an hour," she told Associated Press news agency.
A funeral procession for the children was held at midday, with thousands of people marching through the streets, including hundreds of pro-Fatah security officers, many of whom fired into the air.
Mr Balousheh is considered a leading enemy of Hamas. He was the main interrogator of Hamas members during the 1990s crackdown on the Islamist movement.
Hamas won a landslide victory in elections in January but its funding has been choked off by Western donors because it refuses to renounce violence and recognise Israel.
Mr Abbas has been considering a request by his allies to hold early elections to resolve an impasse in efforts to form a unity government.
Hamas denounced the proposal to hold another election as a "coup against democracy".