Further deadly sewage floods are feared after a wave of stinking waste and mud from a collapsed septic pool inundated a Gaza village, killing five people, including two babies.
The collapse has been blamed on residents stealing sand from an embankment.
It highlighted the desperate need to upgrade Gaza's overloaded, outdated infrastructure - but aid officials say construction of a modern sewage treatment plant has been held up by constant Israeli-Palestinian fighting.
However, construction of a new plant did not appear to have been affected by year-old international sanctions on the Palestinian Authority. The Gaza City mayor blamed the collapse on local people digging dirt from an earthen embankment around the structure and selling it to building contractors.
The existing plant in northern Gaza - located just a few hundred metres from the frontier with Israel - stored incoming waste in seven holding basins. But with the burgeoning population producing nearly four times as much waste as the plant could treat, local officials were forced to store the overflow in the nearby dunes, creating a lake of sewage covering nearly 45 hectares, according to the United Nations.
An embankment around one of the seven holding basins collapsed, sending a wall of sewage crashing into the neighbouring village of Umm Naser.
The wave killed two women in their 70s, two toddlers and a teenage girl and injured 35 others, hospital officials said. More than 200 homes were destroyed, health officials said.
"This is a human tragedy," said Public Works Minister Sameeh al-Abed.
Rescue crews and gunmen from the militant Hamas group rushed to search for people feared buried under the sewage and mud. Most residents fled or were evacuated.
Rescuers in wetsuits paddled boats through the layer of brown foam floating on the green-brown rivers of waste. Others waded up to their hips into the sewage.
Angry residents drove reporters out of the area and mobbed government officials. When Interior Minister Hani Kawasmeh arrived to survey the damage, his bodyguards fired in the air to disperse the crowd.
In one house, everything from the television to the sink was covered in muck. The town was filled with the noxious smell of waste and dead animals.
"We lost everything. Everything was covered by the flood. It's a disaster," said Amina Afif, 65, whose small shack was destroyed.
The collapse will force officials to divert the waste into the other six basins, putting those in danger as well. Another collapse could send sewage flooding into Beit Lahiya, a far larger town nearby, local officials said.
Fadel Kawash, head of the Palestinian Water Authority, said the sewage level had risen in recent days, creeping up the earthen embankments.
Gaza City Mayor Majid Abu Ramadan, who leads a council of Gaza municipalities, blamed the collapse on endemic lawlessness.
He accused local residents of stealing the dirt and selling it to building companies for 300 shekels ($A86.80) a truckload.
A 2004 United Nations report warned that the sewage facility, built to service a population of 50,000, was handling waste from 190,000 people, and flooding was inevitable. It warned that the lake created by the overflow from the seven basins posed a serious health hazard, providing a breeding ground for mosquitoes and waterborne diseases.
Umm Naser is about 300 metres from the border with Israel, in an area where Palestinians have frequently launched rockets into Israel and Israeli artillery and aircraft have fired back. The situation worsened after Hamas-linked militants captured an Israeli soldier last June in a cross-border raid, and Israel responded by invading northern Gaza.
The incident underscored the fragility of the overburdened infrastructure in this impoverished and overcrowded coastal strip of 1.4 million people. The West Bank, too, is suffering from eroding sewage and water infrastructure.