Palestinian Authority President Mahmoud Abbas has urged the international community to intervene over power cuts in Gaza blamed on an Israeli blockade.
Gaza City was plunged into darkness on Sunday after managers shut down the Hamas-run Gaza Strip's only power plant, saying they had no fuel left.
But Israel, which is continuing to provide 60% of Gaza's power, says the Strip still has sufficient fuel stocks.
It accused Hamas of closing the plant as a "ploy" to attract sympathy.
Hundreds of thousands of people's homes - as well as hospitals and factories - were blacked out by power cuts in Gaza City.
Israel cut fuel supplies and closed border crossings into Gaza on Friday in response to continuing rockets attacks from militants in Gaza.
Foreign ministry spokesman Arye Mekel called the blackout "a Hamas ploy to pretend there is some kind of crisis to attract international sympathy."
"If they stop the rockets today, everything would go back to normal," he added.
The UN believes Gaza's 1.5m inhabitants face serious hardship and one of its officials said unheated hospitals were having to rely on generators for operations.
Officials with the European Union, which funds fuel shipments to the Gaza power plant, told Reuters on Sunday that the power station was "very low" on fuel.
"It's only a question of hours," a senior EU official involved in the fuel programme told the news agency.
Mr Abbas, who lost control of Gaza to Hamas last year, called on Israel to "end its blockade of Gaza immediately and allow the entry of fuel to facilitate the lives of the innocent".
He also called for a "special meeting" of foreign ministers of the Arab League to discuss the crisis and threatened to raise the matter with the UN Security Council if Israel did not respond to his appeal.
"We have just shut down the entire power plant," Derar Abu Sissi, a senior official at the Gaza plant, said on Sunday night.
"At least 800,000 people are now in darkness. The catastrophe will affect hospitals, medical clinics, water wells, houses, factories, all aspects of life."
The plant's director, Rafik Maliha, said earlier that the regular fuel delivery from Israel had not arrived because the fuel terminal, Nahal Oz, was closed and the facility had almost no reserves.
The head of the UN agency for Palestinian refugees in Gaza, John Ging, told the BBC it was a desperate situation.
"The hospitals are operating on generator power," he said.
"What it means is that the vital medical equipment is functioning, but there's no power for heating, so it's very, very cold in all the hospitals tonight."
He warned that when diesel supplies ran out, there would be no fuel to power the generators.
A local health official, Dr Moaiya Hassanain, said hospitals faced a catastrophic choice between cutting "electricity on babies in the maternity ward or heart surgery patients" or stopping operating theatres.
Several thousand residents staged a candle-lit march through Gaza City after the blackout.
Shlomo Dror, a spokesman for Israel's defence ministry, insisted the power station had enough fuel to continue functioning.
"If they shut it down, it's not because of a fuel shortage, but because they want to create the impression of a crisis," he said.
He described the closure of the power station as "not comfortable but not a humanitarian crisis".
Israel, which shut the borders on Thursday, has reduced the flow of petrol used in cars and diesel to the strip but says fuel oil and cooking gas are not affected.
More than 200 rockets and mortars have hit Israel from Gaza since an Israeli operation against militants on Tuesday which left 18 Palestinians dead, the military says.
Hamas said its attacks on Israel would not cease because of the sanctions.
"We will not raise the white flag and we will not surrender," spokesman Sami Abu Zuhri told the London-based independent Quds Press web news agency.