Gazans rushed to buy food, fuel and other supplies that have become scarce because of an Israeli blockade - aimed at stopping rocket attacks from Gaza.
Egyptian police took no action to stop people crossing.
Egypt's President Hosni Mubarak says he allowed Gazans in to buy food, but Israel urged Egypt to restore security.
Hamas leader Ismail Haniya called for urgent talks with Egypt and his Palestinian rival, President Mahmoud Abbas, on border crossings.
Map of the Egypt-Gaza border area
"We do not want to control everything, we are part of the Palestinian people," Mr Haniya said, apparently in response to an offer from Palestinian Authority Prime Minister Salam Fayad to control Gaza's borders - so far rejected by Israel.
Hamas has controlled Gaza since last June.
Guns seized
A total of 350,000 Gazans crossed the Egyptian border, Israel's Haaretz newspaper reported.
Hamas has not taken responsibility for breaching the border but quickly moved in to police it, the paper said, confiscating seven pistols from a man returning to Gaza.
"We want to buy rice and sugar, milk and wheat and some cheese"
"This can feed my family for a month," he said.
Correspondents say the breaching of the border is a security concern for Israel, as Egypt is a main source of weapons for the militant groups in Gaza.
But the BBC's Tim Franks in Rafah on the Gaza-Egypt border says it will be difficult for the Egyptians to reseal the border on their own, and Hamas has very little incentive to co-operate.
EGYPT-GAZA BORDER
Palestinians have broken through the border before, in 2005, and it was quickly resealed with barbed wire, but reports say that on this occasion two-thirds of the border wall was destroyed.
Overnight, gunmen set off a number of explosions along the wall near the Rafah crossing.
People then packed into cars and donkey carts, or crossed the border on foot, to buy essential goods.
Among them was Ibrahim Abu Taha, a father of seven, who told the Associated Press news agency: "We want to buy rice and sugar, milk and wheat and some cheese."
One Gaza woman told the BBC as she crossed the border: "We're going over there to our family. They're all there. I haven't seen [them] for 10 years."
Mostly sealed
President Mubarak said he had allowed the Palestinians to come in.
GAZA BLOCKADE
He said he had told Egyptian troops to "let them come to eat and buy food and go back, as long as they are not carrying weapons".
Israeli foreign ministry spokesman Arye Mekel said his government was concerned about the chaos.
But in a BBC interview, he added: "It's the responsibility of Egypt to ensure that the border operates properly according to the signed agreements."
US State Department spokesman Tom Casey said Washington was concerned about the situation, as was Egypt.
In recent months the border has been mostly sealed, in an understanding between Israel and Egypt.
The territory has been short of fuel and other essential goods since last week, when Israel imposed the blockade.
It was eased slightly on Tuesday to allow some fuel and medicines through.
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