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 settlements would remain standing.