The collapse of Egypt's border with the Gaza Strip on Wednesday altered the region's political and security landscape as suddenly as it changed the fortunes of Palestinians who poured out of the enclave to stock up on goods made scarce by an Israeli blockade.

After masked gunmen used land mines to blast through a 7-mile-long border wall, tens of thousands of jubilant Gazans went on an Egyptian spree, buying gasoline, heating oil, rice, sugar, milk, cheese, cigarettes, tires, cement, television sets and cellphones.

But the breach triggered alarm in Israel over the prospect that Palestinian militants could return with weapons, slipping into crowds of shoppers lugging household merchandise to the impoverished territory run by the Islamic movement Hamas.

Israeli leaders were stunned by the sudden turn of events, which gave Hamas at least a temporary victory over the government's effort, backed by the United States and Egypt, to keep Gaza isolated. Hamas planned, or at least tolerated, the barrier's demolition, and its policemen directed the cross-border traffic.

The endeavor to contain and weaken Hamas is a cornerstone of President Bush's strategy for brokering a peace accord between Israel and the secular-led Palestinian Authority, which continues to control the West Bank. Hamas, which advocates Israel's destruction, has allowed near-daily rocket attacks against Israel that threaten to undermine the peace talks.

Now the United States and Israel, which have been pressing Egypt to halt weapons smuggling to Gaza through tunnels, face unsettling new scenarios: a freer flow of weapons across a chaotic, open border; rising Islamic, pro-Palestinian militancy in secular-led Egypt; and a loosening of Gaza's long-term border control arrangements.

"Our concern is not that anyone is going to Egypt to buy food," said Aryeh Mekel, an Israeli Foreign Ministry spokesman. "Our concern is that when Gaza's exits are open, the entrances are open too, for more weapons, explosives and missiles. For us, that could make a bad situation in Gaza a lot worse.

"We expect the Egyptians to solve the problem," he said.

The open border also poses a risk for Egypt: weapons smuggling in the opposite direction, from Gaza to anti-government militants in Egypt's Sinai region.

Egyptian President Hosni Mubarak's government has been under fire from Islamists at home for not doing enough to help the Palestinians. On Wednesday, riot police in Cairo dispersed 2,000 people led by the outlawed Muslim Brotherhood who tried to hold a rally to protest the Israeli blockade.

Egypt's Foreign Ministry and Hamas' leadership used Wednesday's chaos to call for international talks on revising the protocol for controlling Gaza's borders and steps to ease the territory's isolation.

Many of Gaza's 1.5 million residents have suffered shortages of electricity, fuel, medicine and other supplies for months. Gaza has been all but sealed since Hamas seized control of the territory and its border crossings from Palestinian Authority President Mahmoud Abbas' Fatah faction in June.

Israel had further tightened the sanctions Jan. 17, cutting off fuel for Gaza's only power plant in response to a sharp increase in rocket attacks on targets across the border in Israel.

The blockade was eased slightly Tuesday, but not enough to prevent a mass rush on the border after the explosions hours later.

"What happened today is a natural reaction by people who have been living in a pressure cooker of sanctions," said Kadura Fares, a senior Fatah leader in the West Bank. "Add more and more pressure, and the place was bound to explode. Today it exploded in Egypt's direction. Tomorrow it could explode in Israel's direction."

Faced with demands at home to help the Gazans, Egypt's government set aside its own security concerns and withdrew its guards from the border area after a brief, futile effort to contain the Palestinian exodus.

"I told them to let [the Palestinians] come in and eat and buy food and then return later as long as they were not carrying weapons," Mubarak told reporters in Cairo.

Emad Gad, an analyst with Cairo's Al Ahram Center for Political and Strategic Studies, said Mubarak had little choice.

"We have only 750 policemen" at the border, he said. "The alternative was to shoot the Palestinians and leave tens or hundreds wounded, but the Egyptian authorities would not accept such an alternative."

The border bisects the city of Rafah. It was fortified by Israel with concrete slabs and metal sheets after the second Palestinian uprising began in 2000.

Witnesses said Hamas militants on the Gaza side of Rafah sliced through metal sections of the wall with blowtorches a month ago, weakening the structure. About two-thirds of the 18-foot-high partition collapsed before dawn Wednesday when militants set off 17 land mines.

By midmorning, thousands of Gazans had massed at the border and overwhelmed the police.

Many entered Egypt not only to shop but also to reunite with relatives after months of separation. A Gaza cameraman rushed to fetch his Egyptian fiancee and started planning a long-delayed wedding.

At least four Palestinians in wheelchairs were pushed to the Egyptian side, where ambulances picked them up to take them for medical treatment in El Arish, an Egyptian town not far from Gaza.

The open border is "a gift from God," said Ahmed Shaeer, 38, an unemployed Gaza construction worker who was carrying his 18-month-old son to Egypt to buy medicine for the child's liver ailment.

"I'll also get as much food and milk as I can carry home to my family, but I must hurry," he said, walking briskly through the crowd. "I don't believe this freedom will last long."

Egyptian officials gave no indication of when they intended to restore control over the border.

In Washington, White House Press Secretary Dana Perino said the instability was "very troubling" for Israel.

"It is Hamas' actions of lobbing upward of 150 rockets a day into their territory that has caused . . . Israel to implement the blockade," Perino said. "The Palestinians living in Gaza are living under chaos because of Hamas, and the blame has to be placed fully at their feet."

In Gaza, Hamas leader Ismail Haniyeh called for a meeting with Egypt and Abbas' government to work out new arrangements for border crossings, including an end to Israel's authority to prohibit traffic through the Gaza-Egypt crossing.

Haniyeh indicated that Hamas was prepared to cede some control over Gaza's borders, with Egypt and Israel, to Abbas' West Bank-based administration in the hope of easing the blockade.

Abbas said this week that he and the Israelis were discussing his own proposal to regain control of Gaza's borders under an arrangement that would be supervised by the European Union. Egyptian officials have voiced their support, and Israel has said it is studying the idea.

But Hamas' proposal drew criticism from Ashraf Ajrami, a Cabinet minister in Abbas' government.

"Everything Haniyeh is saying is simply to exploit this situation," he told the Associated Press. "It is a part of the problem, not the solution."

Meanwhile, Gaza's power plant, which serves most of Gaza City's 400,000 people, faced the prospect of its second shutdown this week after its supply of industrial diesel fuel pumped from Israel dwindled to one day of reserves. Israeli officials said they had made no commitment to resupply it.

Israeli Prime Minister Ehud Olmert warned that he would not allow Gazans to live ordinary lives while Israelis were enduring rocket attacks.

"We will not allow under any condition, or any situation, creation of a humanitarian crisis," he said in a speech at a security conference. "We will not hit food supplies for children or medicines for the needy."

But he added: "Does anyone seriously think that our children will wet their beds at night in fear and be afraid to go out of the house, and they [Gazans] will live in quiet normality?"