Palestinians have bulldozed down part of the Gaza-Egypt border wall again, hours after Egyptian troops blocked holes recently made by militants.
Guards with riot shields stood by and then pulled out of the border areas completely as Palestinians passed through the newly-demolished barrier.
Hundreds of thousands have surged into Egypt to buy supplies since the first breaches were made on Wednesday.
Recently Israel tightened a blockade of Gaza, leading to acute shortages there.
Israel imposed the embargo after a sharp rise in rocket attacks from Gaza on its southern towns.
The UN has estimated that as much as half of Gaza's 1.5 million population has crossed the border in defiance of the blockade.
The new breach came hours after Egyptian security forces had begun to stop Palestinians from entering their country while at the same time allowing people back into Gaza.
The large yellow bulldozer was driven to the border from the Gaza side, ploughing headlong into the border fence.
Police used water cannon to push people back from the border
The BBC's Aleem Maqbool at the border says Egypt's attempt to seal it is being seen as a minor interruption by Palestinians in search of supplies.
People were continuing to ferry essential goods such as petrol and cooking oil back into Gaza, as they have for the past two days, eyewitnesses said.
The Islamist movement Hamas, which controls Gaza, said it respected Egypt's decision to close the border but it did nothing to assist the Egyptians on the ground.
Israel has demanded Egypt take action, as it is worried about arms smuggling.
But the latest incident is a humiliating setback for Cairo, which must now decide how to respond, the BBC's Ian Pannell, on the Egyptian side of the border, says.
Egypt may now have to consider talks with Hamas, which it has previously ruled out, he adds.
Riot police armed with electric batons attempted to seal the breach, while water cannons were aimed above the heads of the jostling crowd after some Palestinians threw stones.
Live shots were also fired from both sides.
Egyptian border guards meanwhile began placing piles of barbed wire and chain-link fences along the border in an attempt to re-seal it.
Hamas has said it supports Egypt's decision to close the border and denied any involvement in the new breach.
But unless the group agrees to help police the border, it will be very difficult to keep it closed, our correspondent says.
The move by the Egyptian authorities came only hours after the US Secretary of State, Condoleezza Rice, urged them to secure the border with Gaza.
Later, Egyptian foreign ministry spokesman Hossam Zaki promised it would "go back as normal".
Israel has stepped up its security since the border fence was destroyed, with citizens warned against travelling to Egypt's Sinai peninsula.
17 January: Israel seals border following rise in rocket attacks
20 January: Gaza's only power plant shuts down
22 January: Israel eases restrictions
22 January: Egyptian border guards disperse Palestinian protest against closure
23 January: Border wall breached
Israeli officials said police were on increased alert because of warnings about infiltration and fears that militants will acquire more weapons in Egypt.
Overnight on Thursday, the Israeli military carried out two air strikes near the border, killing four suspected Hamas militants.
The commander of the Islamist movement's military wing in Rafah, Mohammed Abu Harb, was killed along with another senior militant when Israeli missiles hit their jeep in the town, Palestinian medics said.
Another two died two hours earlier in an Israeli air strike on a truck in southern Rafah.
Israel has killed more than 40 Palestinians in Gaza during the past 10 days.