JEBALIYA, Gaza Strip (AP) – A truck filled with masked militants and homemade weapons exploded at a Hamas rally Friday, killing at least 15 Palestinians and wounding 80 – including children – bringing a grisly and terrifying end to one of the last gatherings by armed groups celebrating Israel’s Gaza pullout.

The blast sent a huge cloud of white smoke over the mass festivities, a sea of green Hamas flags and thousands of people gathered at Jebaliya, a Palestinian refugee camp that was the scene of harsh fighting between militants and Israeli soldiers during the past five years of violence.

After initial confusion, people began running away from the rally and gunmen fired in the air. People wailed in grief as others tried to tend to the mangled and dismembered bodies of the dead and wounded.

Mishandled explosives apparently caused the blast, which came a day before an agreement by militants not to publicly parade weapons is to take effect.

Witnesses said many children were among the casualties.

Hamas said six militants were killed, including Jihad Shaleal, head of the group’s military wing in Jebaliya. Hamas blamed Israel, but the Israeli military denied any connection. Palestinian security officials said the blast was an accident.

“There was smoke all over, and then we saw people in pieces, but we couldn’t make out what really happened,” said 18-year-old Hazem Abu Rashad.

Since Israel left Gaza last week, ending a 38-year military occupation, militant groups have held rallies throughout the Mediterranean coastal strip. Masked militants paraded with rockets, grenades and rifles in celebrations lauding their campaign of suicide attacks and other violence as having forced Israel’s withdrawal from land Palestinians claim for a future state.

Palestinian leader Mahmoud Abbas, struggling to combat the chaos and the public display of arms in Gaza, wrested a pledge from militant groups to stop holding military-style parades with weapons after Saturday.

“There is absolutely no excuse to parade weapons in the streets,” Palestinian National Security Adviser Jibril Rajoub said Friday. “The (militant groups) are merely trying to express their power and their capabilities. I would hope Palestinian society will soon be rid of all of these images.”

The explosion came in the middle of the celebration. The militants are extremely popular with young Palestinians, and teenagers surrounded the pickup before the blast, said Abu Rashad, who was just a few feet away. He said three militants with two homemade rockets were in the truck’s bed, and three or four other militants rode inside.

Men carried bloody body parts and corpses wrapped in blankets to nearby cars. At Shifa Hospital, doctors treated patients on the emergency room floor after they ran out of beds. Masked Hamas men wheeled in casualties.

Palestinian hospital officials said 15 people were killed and 80 others injured. Dozens of children were wounded in the blast, health officials said.

Hamas swiftly claimed Israeli aircraft had targeted the militants with a missile. “We will avenge the blood of our martyrs,” said Nizar Rayan, a Hamas leader.

But Palestinian officials said the explosion was set off by the mishandling of explosives. The Interior Ministry issued a statement calling on Hamas “to shoulder its responsibility for these … explosions instead of making accusations against others.”

An accidental explosion would be only the latest in a string of deadly mishaps for militant groups in Gaza.

A Hamas weapons warehouse exploded this month in Gaza City, killing six people. Hamas claimed it was an Israeli attack, but Palestinian security forces found the blast was an accident caused by the militants.

During an Islamic Jihad rally at the abandoned Jewish settlement of Netzarim last week, a gunman died after accidentally shooting himself in the head.

Even after the blast Friday, seven or eight gunmen stood in the back of another truck riding through Gaza, using their feet to stop a half-dozen rockets from bouncing around in the bed.

Elsewhere Friday, Israeli forces killed three Palestinian gunmen in a West Bank raid.

Troops entered the village of Ilar near Tulkarem after midnight and surrounded a building to arrest senior Islamic Jihad militants inside. Three gunmen fled and were shot dead after opening fire on pursuing Israeli troops, the military said.

Abbas called the killing a “dangerous and unjustified action. We are exerting efforts to maintain the (February) cease-fire and they are doing this action without any reason.”

Despite the truce, Israel continues to target cells of Islamic Jihad, which has been involved in several attacks on Israeli targets. The group launched four homemade rockets into Israel on Friday afternoon, claiming it was retaliation for the raid. The rockets caused no injuries or damage.

Meanwhile, Palestinians temporarily opened the crossing between Gaza and Egypt, hoping to set a precedent and pressure Israel to reach a permanent border agreement with them.

Israel shut down Rafah, Gaza’s main gateway to the outside world, before pulling out. It wants Rafah sealed for six months for a technological upgrade and to test Palestinian ability to take control. In the meantime, Palestinians are to use an alternative crossing in Israel, to be opened next week.

Israel, however, approved opening Rafah for two days starting Friday, mainly to allow the passage of people seeking medical treatment or studying or living abroad. Several thousand travelers arrived at the once-heavily guarded crossing Friday morning, some sitting on suitcases and napping as they waited.

Thousands of Palestinians broke through the Gaza-Egypt border last week after Israel withdrew and before the frontier was sealed. Mark Regev, an Israeli Foreign Ministry spokesman, said Israel was concerned by the border chaos last week and doesn’t want the crossing permanently reopened until security is upgraded.