Lights have gone out in Gaza City after the territory's only power plant was closed down due to a fuel shortage.
Darkness descended on Gaza as the second of the plant's two working turbines was switched off on the third day of a crippling Israeli blockade of the territory.
Sunday's shutdown has prompted fears of a humanitarian crisis in the Gaza Strip.
Palestinians said the worst affected could be the health sector, with hospitals failing to provide services in the absence of electricity.
"At least 800,000 people are now in darkness," Derar Abu Sissi, general director of the plant, said.
"The catastrophe will affect hospitals, medical clinics, water wells, houses, factories, all aspects of life."
The Israeli foreign ministry said the diversion of fuel supply from domestic power generators to other uses was "wholly a Hamas decision".
"Noteworthy is the fact that while the Gaza population remains in the dark, the fuel generating power to the Hamas rocket manufacturing industry continues to flow unabated," it said in a statement to Al Jazeera.
"The Hamas claim of humanitarian crisis in Gaza is also greatly exaggerated." Mekel said that supplies of petrol used in cars, as well as diesel, had been halted but not fuel oil and cooking gas.
"The ball is in their court," he said. "If they stop the rockets today, everything would go back to normal."
Power outages have become commonplace in the Gaza Strip in recent months after Israel declared the area a "hostile entity" and began restricting fuel supplies.
Ahead of the shutdown, residents bought up batteries and candles, as well as basic foods like rice, flour and cooking oil. Bakeries stopped operating because they did not have power or flour.
UNRWA, the UN organisation supporting Palestinian refugees, warned the shortages would drastically affect hospitals, sewage treatment plants and water facilities.
"The logic of this defies basic humanitarian standards," Christopher Gunness, UNRWA spokesman, said. Patients at risk
Dr Medhat Abbas, head of the crisis management unit at the health ministry in Gaza, said that electricity from generators would only be available for a few more hours at the Al-Nasser children's hospital.
"These patients and these children are facing their destiny and they will die soon," he told Al Jazeera.
"They escaped from their poor houses were they have very cold weather ... The families brought them here to be saved in the incubator. Now the incubator and the nursery will be out of electricity.
"What sort of humanitarian law is this?"
He said the blackout would also deprive cancer and intensive care patients of their treatment as well as spoiling blood and vaccines that were being stored.
Al Jazeera's Jacky Rowland in Gaza said that it was not only power generation that would be affected.
"It also means no fuel for the generators that fuel the water pumps - a lot of the water in Gaza is deep beneath the surface, and it has to be pumped to the surface - so no fuel can also mean no water."
The UN has said Israel should not collectively punish Gaza's population while responding to security threats.
The organisation has criticised Israel's decision to close border crossings into Gaza, preventing aid deliveries to the 1.5 million people living in the territory, saying on Saturday that the move could provoke a humanitarian crisis.
Ban Ki-moon, the UN secretary-general, also urged an immediate end to violence in Gaza and Palestinian sniper and rocket attacks into Israel.
Zeev Boim, an Israeli cabinet minister, said that rather than condemning Israel's move, the UN should condemn Palestinian rocket attacks.
"I don't hear the UN's voice," he said.
Israel has continued to push ahead with its military offensive against Palestinian fighters in both Gaza and the West Bank in recent days.
Late on Sunday, an Israeli air raid killed at least one Palestinian and critically wounded another in the northern Gaza Strip. Hamas officials said that the target was a group of fighters who launched makeshift rockets into southern Israel.
Around 230 such rockets and mortars have been fired over the border since Tuesday, according to the Israeli military. At least 36 Gazans have been killed by Israeli fire in the past week.
However, Arye Mekel, an Israeli foreign ministry spokesman, said that the shutdown was unnecessary. "They have an interest in exaggerating," he said.
Israel says the blockade imposed on Gaza is in response to rockets being fired from the territory.