Egypt started to close its breached border with the Gaza Strip on Friday but Palestinian militants bulldozed a new opening in a challenge to Cairo and Israel's blockade of the Hamas-run territory.

Palestinian crowds cheered as Hamas militants used a bulldozer to flatten sections of the chain and concrete fence. In a scene broadcast live on television around the world, Egyptian riot police watched from a distance as hundreds of people poured into Egypt.

Tens of thousands of Gaza Palestinians have been pouring into Egypt stock up on food and fuel in short supply because of an Israeli blockade since militants blew up a border wall on Wednesday.

The fall of the Rafah wall punched a new hole in a U.S.-backed campaign to curb the clout of Hamas and strengthen Palestinian President Mahmoud Abbas, nearly eight months after the Islamist group routed Abbas's Fatah forces in Gaza.

Pressed by the United States and Israel to take control of the situation, Egyptian forces in riot gear lined the border and began placing barbed wire and chain-link fences to prevent more Gazans from entering Egyptian soil.

Egyptian security forces told the crowd over loudspeakers that the border would close at 3 p.m. (8 a.m. EST), but a security source said orders had yet to be given to fully seal the area.

Tensions flared as some Palestinians threw stones at the police, who responded with batons and water cannon.

"I have two brothers still inside Egypt. They should not close the border until everyone returns," said one Palestinian stone thrower, 20-year-old Mohammed al-Masri.

The Egyptian government faces a difficult balancing act. It does not want to be seen as aiding Israel in its blockade of Gaza, but it fears the spread of Islamist influence and the effects of becoming home to so many undocumented Palestinians.

Israel said it had tightened its Gaza blockade last week to counter cross-border rocket fire, but after an international outcry, fuel and aid supplies were partially restored.

Egyptian President Hosni Mubarak, in an interview to be published on Saturday, urged Hamas and Fatah to end their differences and invited both sides to meet for talks in Cairo.

Abbas, who is also leader of Fatah, has been seeking U.S. and Israeli support to take over control of all of the border crossings, a move Hamas hopes to prevent.

STANDOFF AT BORDER

By challenging Egyptian efforts to re-close the Gaza border, Hamas hoped to win assurances from Cairo that it would have a say in any future agreement to oversee the border crossings, including the one with Egypt at Rafah, Hamas sources say.

Israeli officials said Abbas, whose authority is largely limited to the West Bank, planned to meet Israeli Prime Minister Ehud Olmert on Sunday, seeking support for controlling the crossings and for renewed peace talks despite the setbacks.

Citing the breach in Gaza's southern border, some top Israeli officials have advocated cutting Israel's remaining links with the coastal territory and putting the onus on Egypt.

Hamas sources said the group decided to open a new section in the border fence to increase pressure on Egypt.

"We insist and urge our Egyptian brothers that there must be a mechanism to allow the passage of people and goods through the Rafah crossing in a legal and organized manner," Hamas government spokesman Taher al-Nono said.

Israel, which occupied Gaza in 1967, pulled troops and settlers out in 2005, but it still controls the strip's northern and eastern borders, airspace and coastal waters.

Earlier on Friday, Israel killed four Palestinian militants in an overnight air strike on Rafah.

Violence has also flared in the occupied West Bank, where Israeli troops shot dead a Palestinian teenager one day after militants killed an Israeli border policeman and infiltrated a Jewish settlement near Bethlehem.

Hamas's armed wing claimed responsibility for the infiltration in which the two Palestinian attackers were killed.