Palestinian gunmen fired a barrage of homemade missiles from the Gaza Strip into southern Israel before dawn Saturday, wounding five Israeli civilians, a military official said. Attack helicopters responded hours later by striking a weapons factory and warehouse in Gaza, the first Israeli military operation inside the strip since Israel completed its withdrawal from the area this month.

The operation followed a horrific blast Friday in the Jabalya refugee camp north of Gaza City. A truck carrying a group of armed men and makeshift weapons exploded during a Hamas rally, killing at least 15 people and wounding more than 80, according to hospital officials and witnesses.

Hamas, formally known as the Islamic Resistance Movement, accused Israel of firing missiles at the vehicle from an unmanned aircraft, which Israeli military officials denied. Witnesses said the explosion did not leave the telltale crater of a missile strike, a method Israel has used frequently against Hamas leaders and gunmen in the past.

Palestinian security officials said they believed homemade explosives in the truck may have ignited, sending shrapnel slicing through a dense crowd of several thousand people.

The wounded filled local hospitals throughout the late afternoon, and medical officials said many of the victims were children.

The rally was one in a recent series of demonstrations called by leaders of Hamas, a radical Muslim movement, to celebrate Israel's departure from the Gaza Strip this month.

The Israeli military operation early Saturday followed rocket attacks carried out by Islamic Jihad, a smaller Palestinian faction. Israeli attack helicopters fired missiles into what an Israeli military spokesperson described as a weapons factory in the Gaza town of Zeitun and weapons warehouses in Jabalya and Gaza City. There were no immediate reports of casualties.

Israeli military officials said Islamic Jihad fighters fired more than 20 rockets toward the southern Israeli city of Sderot before dawn, by far the largest of three rocket attacks from Gaza over the past two days. Most of the homemade rockets, notoriously inaccurate, fell into vacant fields. But at least one of them wounded three civil guard volunteers in Sderot, one of them moderately. Two other Israeli civilians were also wounded, but none appeared to have suffered life-threatening injuries.

The group's leaders said the rocket attacks were a response to a pre-dawn Israeli military operation early Friday near the West Bank city of Tulkarm. Israeli soldiers killed three men whom military officials identified as Islamic Jihad gunmen responsible for shooting attacks on soldiers and Israeli settlers in the area.

"We hold the Palestinian security forces responsible for preventing all terrorist attacks originating in Gaza against Israelis," the Israeli military spokesperson said.

Special correspondent Islam Abdel Kareem in Gaza City contributed to this report.

Smoke rises from a blast at a rally organized by Hamas. A truck carrying armed men and makeshift weapons exploded, killing at least 15 people, officials said. A Palestinian weeps beside an ambulance after a blast at the rally Friday. Several thousand people had gathered in the Jabalya refugee camp.