The only power plant in the Hamas-run Gaza Strip has shut down because of a lack of fuel, Palestinian officials say, blaming Israeli restrictions.
Cars were still driving along Gaza City's darkened streets
Gaza City was plunged into darkness after the plant's turbines stopped.
Israel's closure of border crossings amid continued rocket fire from Gaza has brought the delivery of almost all supplies, including fuel, to a halt.
But Israel, which provides 60% of Gaza's power, says the territory still has sufficient fuel stocks.
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.
Mahmoud Abbas, the West Bank-based Palestinian Authority president 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".
'Very, very cold'
"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.
GAZA'S ELECTRICITY SOURCES
Gaza uses 200 megawatts of electricity
Israel supplies 60% of this, and Egypt 8%
The remaining 32% is produced by Gaza's power station
Israel supplies the fuel oil for the Gaza power station
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.
Israel says border closures will stop if the rocket attacks end
"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.
Israeli foreign ministry spokesman Arye Mekel said the border closures were intended to apply pressure to the Palestinian authorities to stop militants in Gaza firing rockets at Israel.
"If they stop the rockets today, everything would go back to normal," he said.
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.
Israeli ministers meeting on Sunday decided to maintain the border closure for the time being, an unnamed source told AFP news agency.
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.