( BBC ) - 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.
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 says Gaza's 1.5m inhabitants face serious hardship.
Reports from Gaza say people are trying to stock up on candles and batteries, as well as basic foodstuffs.
One grocer in Gaza City, Sami Mousa, told the Associated Press the shopping fever would be worse except that "people don't have the money to buy".
"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."
Several thousand residents staged a candle-lit march through Gaza City after the blackout.
The Palestinian Energy Authority (PEA) had begun shutting the plant's turbines early in the morning.
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 closure comes amid the peak winter demand for electricity.
Christopher Gunness, spokesman for the UN relief agency UNWRA, predicted the closure of the power station would "have a significant impact on the daily lives of hundreds of thousands of people in Gaza".
Most Gazans are reliant on fuel imports and humanitarian supplies.
One pharmacist in Gaza, Maher Abu Halawa, told the BBC he was quickly running out of supplies.
"All this affects patients, especially the diabetes patients whose medicine has completely run out," he said.
"Medications needed for cancer patients have also run out completely... We have also run out of children's medications, particularly vaccines."
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.
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.