As children were going to school in the Ramal district of Gaza City, armed men fired on a car killing the sons of Colonel Balousha, a senior Palestinian intelligence officer and Fatah loyalist.
There were no claims of responsibility. The boys were aged between six and eight.
Relatives carried the boys, whose bodies were wrapped in sheets, in their arms.
The mourners marched through the streets of the city escorted by hundreds of pro-Fatah security officers.
|Candles around a poster of images of the|
three young brothers killed in Gaza City
A senior Palestinian intelligence official in the occupied West Bank condemned the latest attack.
He said it was unclear who was behind it.
Besides internal political unrest, Gaza is also riven with clan fighting and a surge in criminal violence since the imposition of a Western aid embargo on the Hamas government that has increased poverty.
An adult was also killed and four other people were wounded in Monday's drive-by shooting.
Fadwa Nabulsi, 12, who witnessed the scene, said: "I was walking with my young brother, Wael, when shooting took place... We started screaming and children started running.
"I lost Wael ... but I found him hiding in a falafel shop. I'm trying to find my father to take us back home."
Palestinian police tried to calm children at the scene. After news of the shooting spread, hundreds of parents went to Gaza City's Shifa hospital.
|Mourners heading for the boys' funeral on|
Monday [Picture: Laila El-Haddad]
After the incident, Fatah supporters tried to close Palestine Street, which has nine schools on it, shouting: "God help us take revenge against the killers."
Elsewhere in the city, other Fatah supporters were blocking off main roads with burning tyres. Some have called for a general strike.
While Gaza has regularly suffered violence, a direct attack on children is unprecedented.
Tension in Gaza has heightened in recent weeks after Mahmoud Abbas, the Palestinian president, raised the possibility of calling early parliamentary and presidential elections.
"Look at the children of the Palestinians. They fight Israel with nothing more than a rock in each hand.. those are the leaders of the Palestinian government"
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Khalid Abu Hilal, a spokesperson for the Palestinian interior ministry, has described the killing of the children as "ugly and cruel".
He said: "The only thing we can understand from Monday's cruel crime, which targeted innocent Palestinian children, is that those who carried it out targeted the internal security and wanted to spark strife, security disorder and chaos in the country."
Abu Hilal said the interior minister has ordered internal security and interior ministry paramilitary forces to arrest the culprits and open an investigation.
Regarding the attack by mourners on the parliament building in Gaza City, Abu Hilal said: "It is clear that, over the last few days, a lot of criminals have been targeting the Palestinian street.
"During this period, the interior minister has achieved success in such areas as storming drugs dealers' dens, ending attacks on Palestinian citizens' lands, and dissolving car-stealing gangs."