Four days into an Israeli blockade that has cut off food and fuel to the Gaza Strip, residents of the strip contemplated Monday how long it would be until disaster hit. One family of 13, shivering in the cold, counted its eight remaining candles. A bakery that normally feeds thousands had three days' worth of flour.

Hospital generators with enough fuel for three days and no spare parts powered incubators in which twin boys born 2 1/2 months prematurely were being kept alive, their thin chests heaving convulsively.

Israel agreed Monday to allow in a one-time shipment of fuel, food and medicine on Tuesday, after Egyptian President Hosni Mubarak telephoned Israeli Defense Minister Ehud Barak to appeal on behalf of Gaza's 1.5 million people. But Israel gave no indication when it planned to fully lift the blockade, imposed Friday in response to escalating rocket attacks from the Palestinian territory.

In the neonatal intensive care unit of Gaza City's main hospital Monday, physician Radwan Hassouna tapped the plastic incubator of Karam al-Namy, one of the 1 1/2 -pound twins, and ticked off the electrical equipment keeping the baby alive.

"He has a ventilator. He has an oxygenator. He has photo therapy," or light to keep him from developing jaundice, Hassouna said. An intravenous pump, he added. And monitors.

If the generators broke down, Karam and his twin brother, Kareem, would die in an instant, Hassouna said.

Leaving the hospital after seeing his young sons, Ashraf al-Namy said he feared he would never see them alive again. "I'm afraid when the electricity goes off," he said. "They only live on artificial respiration. What is going to happen to them?"

Israel closed the border crossings into Gaza on Friday to enforce its demand that the armed Hamas movement that controls Gaza bring a halt to rocket attacks into Israeli territory.

From Tuesday to Friday last week, more than 150 rockets were fired from Gaza. None caused any fatalities, although Palestinian gunmen killed an Ecuadoran farmhand working in a field near Gaza on Tuesday, the day Israeli forces unleashed large-scale ground and air assaults against targets in the northern part of the strip.

Israeli military operations from Tuesday to Sunday killed more than 30 people in Gaza, most of them gunmen, Palestinian officials said.

"As far as I'm concerned, Gaza residents will walk, without gas for their cars, because they have a murderous, terrorist regime that doesn't let people in southern Israel live in peace," Israeli Prime Minister Ehud Olmert told lawmakers from his Kadima party earlier Monday.

Rocket attacks into Israel have declined from more than 30 a day last week to five on Sunday. By late Monday, at least eight rockets and mortar shells had landed.

Israel has limited the flow of supplies to Gaza since Hamas seized power here in June, routing Palestinian Authority President Mahmoud Abbas's Fatah movement, Hamas's partner in a short-lived unity government. Stores of medicine, fuel and other staples had dwindled even before the blockade, Palestinian doctors and engineers here said.

Shlomo Dror, spokesman for Israel's Coordinator of Activities in the Territories, has said repeatedly since Friday that Israel would not allow a humanitarian crisis to occur in Gaza.

By Monday, however, the people of Gaza had had more than a glimpse of what such a crisis would look like.

Gas stations had closed, having exhausted their fuel supplies. Some bakeries had run out of flour.

The U.N. Relief and Works Agency, which distributes food rations to 860,000 Palestinian refugees in Gaza, said Monday that without fuel it would have to suspend operations by Friday. The World Food Program, whose rations help feed another 270,000 Gaza residents, said it would have to stop distributions by Thursday unless it gets more fuel.

The Gaza Strip's power plant, which supplies electricity to about 500,000 people in Gaza City and elsewhere, ran out of fuel Sunday night and was shut down, Palestinians in charge of the electrical system said.

The power plant provided about 25 percent of Gaza's total electricity.

Five power lines from Israel supply another 70 percent, Palestinian and Israeli officials said. Except for one power line damaged before this week, the Israeli power supply to Gaza has not been affected, Israeli and Palestinian authorities said.

However, there is little connection between the grids that carry the electricity from the power plant and the grids that carry Israeli power, said Rafiq Maliha, the project manager of the power plant.

Additionally, the Gaza City network is operated manually, requiring a worker to throw a breaker to connect each area, Maliha said. That makes it difficult to feed electricity from other networks to the one that had been supplied by the power plant.

Power at the main Gaza City hospital came on without notice Monday afternoon, for the first time in about 18 hours. Gaza City moved in and out of blackout through Monday night.

"Gaza is dying slowly," said Ahmed Bahar, a Hamas official, with "an international silence, an Arab silence."

On Monday, Hamas officials urged neighboring Egypt to open its Rafah crossing into the Gaza Strip to allow supplies to enter. Egypt has kept the crossing closed since June, in solidarity with Abbas's government in the West Bank.

There were signs Monday that the blockade was eroding popular support for Hamas.

The movement organized a march by children and medical workers Monday to protest the blockade. The march consisted almost entirely of young boys waving Palestinian flags as they streamed through Gaza City alongside a few Hamas officials and other adults.

Few people along the route joined the march or even appeared to look up as they went about on foot, bicycles and donkey carts, or in the relatively few automobiles still on the road, to search for food.

Wooden stands with cauliflower, tomatoes and other goods grown in Gaza still appeared on street corners. Boys with donkey carts offered fruit brought in earlier from Israel. Meat was twice the price it was 10 days ago.

With some bakeries closed, the normal five-minute wait for bread grew to an hour at one of the bakeries open Monday.

"People say this all started after George Bush visited," observed Hisham al-Ashrami, 31, speaking over his shoulder as he scooped freshly baked loaves off a conveyor belt and into his customers' plastic sacks. The line of people waiting for bread snaked out the door. "They say he gave Israel the green light," Ashrami said.

Other Gaza residents echoed his comments, suspecting a link between Bush's visit to the Middle East this month and the Israeli crackdown on Gaza.

"Why do you think that is?" Ashrami said. "These are all civilians here," he said, gesturing at the bundled-up men, women and children crowded before his bread trays.

He estimated the bakery had enough flour left for three days.

Haya al-Serraj, 25, left the shop with a sack stretched to bursting by loaves of bread for her extended family of 13.

The family still has enough food but only two boxes of candles -- eight in all, Serraj said.

"Enough for two days, I hope," Serraj said cheerfully, then shook her head. "I don't think so."

Serraj plays games with her brother and sister, ages 2 and 3, to distract them during nights without heat, lights or TV, she said. Last week, when Israeli airstrikes were heaviest, she tried to soothe the children, she said.

"But you can tell, as much as you sing them songs and play with them, they must still be starting to figure out what's going on," she said. "They already know. They know this isn't normal."