Updated

Gaza's only electrical plant shut down Sunday after Israel blocked the shipment of fuel that powers them, plunging Gaza City into darkness and sending already beleaguered Gazans to stock up on food and batteries in anticipation of long, dark, cold days ahead.

Israel blockaded Gaza Thursday as a pressure tactic against militants who have been firing rockets at Israel every day. The stricken power plant generates about one third of Gaza's electricity. The rest, which comes from Israel, was not affected by the blockade, Israeli officials said.

A U.N. agency and human rights groups condemned Israel, but Israel said they should direct their criticism at Palestinian militant groups that fire rockets at southern Israel every day.

Israel sealed all crossings into Gaza last week because of a spike in rocket barrages, cutting off fuel. Several weeks ago Israel reduced the supply as a pressure tactic.

In addition to the fuel it receives from Israel to power its electrical plant, Gaza gets about two-thirds of its electricity directly from Israel. Israeli officials said that supply would not be affected.

Hamas officials shut down the plant and plunged Gaza City into total darkness, Gaza Energy Authority head Kanan Obeid said. TV crews and reporters were invited to witness the shutdown just before 8 p.m. (1800 GMT). A candlelight protest march was planned for later Sunday.

The regular fuel shipment from Israel did not arrived Sunday because the fuel terminal was closed, and the plant has nearly no reserves, said Rafik Maliha, director of the power plant.

Health Ministry official Dr. Moaiya Hassanain warned that the fuel cutoff would cause a health catastrophe. "We have the choice to either cut electricity on babies in the maternity ward or heart surgery patients or stop operating rooms," he said.

Shlomo Dror, spokesman for Israel's Defense Ministry, said Gaza has enough fuel and accused Palestinian officials of trying to create the impression of a crisis that did not exist.

Residents of Gaza City were buying up batteries and candles, as well as basic foods like 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 of the blockade, bakers said, because they had neither power nor flour.

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.

Earlier, Obeid called on people to cut back their use of electric appliances. The U.N. organization in charge of Palestinian refugees, UNRWA, warned the Israeli blockade would drastically affect hospitals, sewage treatment plants and water facilities.

"The logic of this defies basic humanitarian standards," said Christopher Gunness, spokesman for the U.N. Relief and Works Agency.

Human rights groups also condemned the fuel cutoff. The British group Oxfam called it "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."

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.

Israeli Cabinet minister Zeev Boim said that rather than condemning Israel's move, the U.N. should condemn Palestinian militants for subjecting Israeli civilians to barrages of rockets. "I don't hear the U.N.'s voice," Boim said.

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.

By nightfall Sunday, four rockets exploded in Israel through 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.

Despite its seven-month blockade, up to now Israel allowed basic food items and humanitarian supplies into Gaza. That changed Thursday, when Defense Minister Ehud Barak ordered all crossings closed after intense rocket fire.

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.