"We have the choice to either cut electricity on babies in the maternity ward or heart surgery patients or stop operating rooms," Gaza Health Ministry official Dr. Moaiya Hassanain said.
Israel justified the cutoff because of continuous rocket attacks by Gaza militants. Israeli Foreign Ministry spokesman Arye Meckel said the Gaza Strip continues to receive 70 percent of its electricity supply directly from Israel, which would not be affected, and another 5 percent from Egypt.
The blackout "is a Hamas ploy to pretend there is some kind of crisis to attract international sympathy," he told The Associated Press.
Late Sunday, Palestinian President Mahmoud Abbas appealed to Israel to lift the blockade, said Abbas spokesman Nabil Abu Rdeneh. Abbas effectively rules only the West Bank after Hamas expelled his forces from Gaza last June.
The exiled leader of Hamas appealed to Abbas and Arab leaders, asking them to forget their differences and help the beleaguered Gazans.
"All Arab leaders, exercise real pressure to stop this Zionist crime ... take up your role and responsibility," Khaled Mashaal told Al-Jazeera satellite TV in a live interview from Syria, where he lives in exile. "We are not asking you to wage a military war against Israel ... but just stand with us in pride and honor."
Officials from the ruling Islamic militant group Hamas shut down the plant just before 8 p.m. and Gaza City went dark, Gaza Energy Authority head Kanan Obeid said. TV crews and reporters were invited to witness the shutdown.
Minutes later, residents started a candlelight march as a protest. Live Associated Press TV pictures showed dots of light moving slowly up a darkened main street.
Israel has blockaded Gaza for seven months, since the Islamic militant Hamas overran the territory, allowing up until now only basic food items and humanitarian supplies into Gaza.
That changed Thursday when Defense Minister Ehud Barak ordered all crossings into Gaza closed because of a spike in rocket barrages, cutting off fuel supplies. Several weeks ago, Israel reduced the fuel supply as a pressure tactic.
A defiant 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, " Hamas spokesman Sami Abu Zuhri said Sunday.
The regular fuel shipment from Israel did not arrive Sunday because the fuel terminal was closed, and the power plant has almost no reserves, said Rafik Maliha, director of the power plant.
The U.N. organization in charge of Palestinian refugees warned the blockade would drastically affect hospitals, sewage treatment and water facilities.
"The logic of this defies basic humanitarian standards," said Christopher Gunness, spokesman for the U.N. Relief and Works Agency, or UNRWA.
The British group Oxfam called Israel's cutoff "ineffective as well as unlawful." Gisha, an Israeli group that has fought the fuel cutbacks in Israel's Supreme Court, said: "Punishing Gaza's 1.5 million civilians does not stop the rocket fire. It only creates an impossible 'balance' of human suffering on both sides of the border."
Israeli Cabinet minister Zeev Boim said that rather than condemning Israel, the U.N. should condemn Palestinian militants for firing rocket barrages at Israel.
"I don't hear the U.N.'s voice," Boim said.
Meanwhile, Gaza City residents were busy buying up batteries and candles, as well as basic foods such as rice, flour and cooking oil, said grocery store owner Sami Mousa. More would be doing the same, he said, but "the problem is that the people don't have the money to buy."
Bakeries stopped operating because they had neither power nor flour, bakers said.
The Hamas-linked Popular Resistance Committees threatened to break the blockade by crashing through the border with Egypt "by force."
There were no signs of panic, as Gazans have been living with fuel cutbacks, power outages and shortages since Islamic Hamas militants overran the seaside territory in June, triggering international sanctions.
Israel pulled out of Gaza in 2005, but many see Israel as still responsible, since it controls most land, sea and air access to the territory.
Alon Ben-David, military analyst for Israel's Channel 10 TV, said Israel could not maintain the blockade for more than a few days.
"Israel understands that a humanitarian crisis is developing here," he said.
Four rockets exploded in Israel throughout the day, a significant drop from the level of last week. The military said since last Tuesday, the start of the escalation, more than 200 rockets and mortars hit Israel.
There have been no serious injuries over the past week, but residents of the Israeli towns have been traumatized by months of daily salvos.
Addressing the annual Herzliya Conference on security, the Israeli military commander, Lt. Gen. Gabi Ashkenazi, said his forces have exacted "a heavy toll" from the militants, but he acknowledged a solution to the rocket fire is not near.
"Even if it takes time, the goal will be achieved," he pledged.
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Associated Press Writer Steven Gutkin in Jerusalem contributed to this report.
