Hamas militants have destroyed a section of a concrete barrier erected along the Gaza-Egypt border.
| Palestinians watched as the explosions took place|
Palestinian and Egyptian troops have been trying to shore up the barrier to stop Palestinians crossing into Egypt after the withdrawal of Israeli troops.
In chaotic scenes, thousands of Palestinians have streamed over the border in the last few days without undergoing official checks.
Despite this, Egypt says that its Gaza border is officially closed.
Militants from Hamas cleared an area before setting off explosives that blew away a section of the wall. Palestinian security officers present did nothing to prevent them.
A local Hamas commander warned them not to try to intervene, AFP news agency reported.
The Egyptian authorities set a deadline of 1800 (1300 GMT) for people to return to the right side of the Gaza border or face arrest.
The mass crossings have raised questions as to whether Egypt can honour its deal with Israel and maintain security along the border, correspondents say.
Many of those crossing have been going to visit relatives stranded on the other side, to buy cheaper food and other goods, or just out of curiosity.
Israel fears militants will exploit the situation to smuggle weapons into Gaza.
Egypt's ambassador to Israel, Mohammed Asim Ibrahim, has vowed to maintain law and order.
But the diplomat expressed sympathy for Palestinians from Gaza crossing the border.
"You're talking about people who have been physically in prison for the last 38 years. So some elements just rushed to the other side of the border," he added.
Symbol of resistance
In a separate development, the Palestinian Authority demolished a synagogue in the Kfar Darom settlement in Gaza.
It was here that many hundreds of Jewish settlers and opponents of the Gaza disengagement plan barricaded themselves on the roof in a last-ditch attempt to resist eviction.
The BBC's Matthew Price in Kfar Darom says the settlement has become one of the symbols of settler resistance to Israel's evacuation plan.
The Israeli cabinet had earlier voted that more than 20 synagogues at the evacuated settlement would remain standing.
MIM: How convienent that the news media was out in force when Egyptians suddenly 'discovered and destroyed' an arms smuggling tunnel filled with 'weapons and grenades', while Hamas blew a hole in the border wall and thousands of Arabs were streaming back and forth unchecked. Even more perversely amusing is that journalist Tom Perry describes the destruction of the tunnel without even alluding to the distinct possibility that the Egyptians knew of the tunnel all along. Perry dutifully reported that the blatantly obvious for media consumption tunnel demolition was " a reassuring action for Israel".
By Tom Perry
RAFAH, Egypt, Sept 14 (Reuters) - Egyptian border guards destroyed a tunnel to the Palestinian-ruled Gaza Strip after finding guns and grenades in it, a reassuring action for Israel two days after it left Gaza.
"The opening of a tunnel was found ... and it went to the other (Gaza) side of the border," an Egyptian security official, who asked not to be identified, told Reuters on Wednesday. He said it was discovered near the Rafah border crossing.
Israel says Palestinians have long smuggled arms from Egypt.
Egypt has begun deploying 750 border guards along the 12-km (seven-mile) border with Gaza under an agreement to help Israel end 38 years of control of the strip, home to 1.4 million Palestinians.
Israel left on Monday but still controls Gaza's sea lanes, air space and other borders.
The tunnel, which started inside a house and was at least 10 metres (33 feet) below the surface, contained 35 automatic rifles with 115 magazines and three rocket-propelled grenade launchers with 280 grenades, the security official said.
He said no one was found in the tunnel or nearby, and the tunnel had since been destroyed.
Since Egyptian guards started deploying on Monday, thousands of Palestinians have crossed the frontier freely into Egypt, which says it wants to let them celebrate the Israeli withdrawal before resealing the border.
Israeli government spokesman Avi Pazner said Israel had confidence in Egypt's ability to control the frontier and stop arms smuggling.
On Wednesday, two members of the Palestinian guerrilla group Islamic Jihad with automatic rifles drove on a road between the border fences in a Mercedes, flying Islamic Jihad flags.
"As soon as they (the Israelis) withdrew, we came across to see the people," one of those in the car, who declined to be identified, told Reuters.
Dozens of people were still crossing either way on Wednesday through breaks in the fence. Some carried food and one man was leading a goat back to Gaza.
Many Palestinians who live next to the border have relatives on the other side of the fence.
At some points along the fence, which is about 10 metres (33 feet) high, Palestinians had erected make-shift ramps and ropes to clamber over.
Egyptian officials said people would be allowed to return to their own sides by 6 p.m. (1500 GMT). It was not clear what would happen if any missed the deadline.
"Our instructions are not to allow anyone to come or go after 6 p.m.," said one Palestinian guard, who asked not to be identified, as he stood near a break in the fence.
There were no Egyptian guards at that opening or at some other breaks in the fence where people passed through.
Egypt has said it will complete the deployment of its 750 border guards on Thursday. Prior to signing an agreement on the deployment, Egypt had been restricted to patrolling the frontier with lightly armed police.
Once Egypt seals the border, it is not clear where Palestinians will be able to cross. Israel wants them to travel to a crossing point south of Gaza, diplomats say. (Additional reporting by Yusri Mohamed in Ismailia, writing by Edmund Blair in Cairo)