Much of Gaza City fell into darkness Thursday night after an Israeli blockade, tightened in response to Palestinian hostilities, caused the city's electricity plant to run critically low on fuel and shut down.

Israel also barred 30 truckloads of relief supplies from entering the Gaza Strip, leaving a United Nations agency without food to distribute to needy families that make up half the Palestinian territory's 1.5 million people.

The partial blackout and the food shortage were the most severe consequences of recent hostilities that have shattered a 5-month-old cease-fire along Israel's border with Gaza. With the cease-fire accord due to expire next month, Israel and Hamas, the Islamic group that governs Gaza, appeared to be bracing for another round of heavy fighting.

Hundreds of Palestinians in Gaza City joined a candlelight march, organized by a Hamas-backed group, to protest what they called an Israeli siege.

"This is a crime against innocent civilians," said Ziad Abu Khousa, 23, who wondered aloud how he and other students at Gaza City's Islamic University could manage to study for midterm exams without lights at night. "Half the population of Gaza are women and children, and they have nothing to do with the fighting."

Many refugees of the 1948 war over Israel's creation and their descendants live in shantytowns, and nearly all depend on the U.N. for flour, sugar, rice, canned meat and other staples, delivered from Israel.

Neither the Israelis nor the Palestinians have renounced the cease-fire. But Hamas has permitted sporadic rocket fire into Israel, and Israeli forces this month have made two brief pinpoint raids into Gaza, killing 10 militants.

As required by the cease-fire deal, Israel in June began easing a blockade it had imposed last year when Hamas took control of the Gaza Strip after the collapse of a powersharing deal with Fatah, a more moderate, U.S.-backed Palestinian group.

But on Nov. 4, after discovering a cross-border tunnel dug by Gaza militants, Israel shut down the commercial crossings where basic goods pass into Gaza. Israeli authorities allowed in a limited amount of fuel on Tuesday.

John Ging, director of the U.N. Relief and Works Agency operations in Gaza, said the Defense Ministry had promised late Wednesday that it would allow the 30 truckloads of food and other humanitarian supplies to cross Thursday.

But at midday Thursday, Defense Minister Ehud Barak ordered the border crossings to remain closed. The truck convoy was turned back, ministry spokesman Peter Lerner explained, because of "continued rocket fire and security threats at the crossings."

Palestinian militants fired eight rockets and several mortar shells into southern Israel during the day. No one was wounded in the attacks.

Amos Gilad, a senior Defense Ministry official, said the main concern behind the decision was an intelligence report that Gaza militants planned to attack one of the border crossings, which are guarded by Israeli soldiers.

"Israel is working to prevent the occurrence of a humanitarian crisis in Gaza but will not endanger its soldiers," Gilad told Israel Radio.

For the same reason, Israel halted a planned delivery Thursday of fuel to the Gaza power plant, which supplies electricity to about two-thirds of Gaza City's 480,000 people. The 140,000-megawatt plant runs on diesel purchased in Israel by the European Union and shipped across the border.

The rest of the city and Gaza Strip run on electricity bought directly from Israel and delivered over power lines not affected by the tightening of the blockade.

Kanaan Ebed, deputy head of the Palestinian Power Authority, issued an appeal to Western governments to press Israel to allow fuel delivery to the plant.

Large swaths of Gaza City went dark after the last generator shut down about 7 p.m. Candles burned in most homes. But about half the affected shops cranked up generators, fueled by diesel smuggled from Egypt.

"We were starting to feel some power cuts last week, and we managed to buy some Egyptian fuel," said Bassam Abu Amsha, 35, standing behind the counter of his lighted pastry shop. "But it's only enough for a couple of days."

Egypt, cooperating with Israel, has kept its border with Gaza closed. The easing of the blockade under the cease-fire terms was applied only at Israeli crossings.

But smugglers have dug dozens of tunnels under the Egypt-Gaza border. Many Gaza businesses, hospitals and homes with generators have stocked up on large plastic containers of Egyptian diesel.

Ging said his U.N. agency distributed the last of its stocks Thursday, leaving it unable to continue the handouts Saturday unless the trucks get through. (The agency does not hand out food on Fridays.) He said the United Nations, which has been distributing food in Gaza for 60 years, had never run out of supplies.

"This is a tragic situation, a man-made crisis," Ging said. "I understand the security challenge at the border, but this challenge has to be overcome. International law requires that civilians in a conflict zone must have access to the goods they need to survive."

Ging said he was in "continuous contact with the Israeli government, but they're unable to give us any clarity on when we'll get some relief."