Israeli forces attacked suspected rocket launching sites in the northern Gaza Strip on Friday, and one artillery barrage killed seven Palestinian civilians, including three children, at a family picnic at the beach.

The violence, which also included a missile strike that killed three Palestinian militants, fueled anger already heightened by the death of a top militant commander in the Hamas-led government in an airstrike Thursday.

Tens of thousands of people, including angry gunmen defiantly firing in the air, packed a southern Gaza soccer stadium for the man's funeral.

CountryWatch: Israel

The death toll in Gaza was the highest since Hamas took office in March following its victory in legislative elections.

The Israeli army said it had targeted areas in the northern Gaza Strip used by Palestinian militants to fire homemade rockets at Israel. But one artillery strike appeared to go dramatically off course.

The shells struck a large crowd of people at a beachside picnic, killing seven people and wounding more than 30 others, Palestinian Health Minister Bassem Naim said.

A woman and two young children, six months old and 18 months old, as well as a young teenager were among the dead, medical officials said. All of the dead were believed to be related.

The barrage scattered body parts along the beach, destroyed a tent and sent bloody sheets flying into the air. A panicked crowd quickly gathered, screaming and running around in confusion.

One young girl screamed hysterically and threw herself on the sand. "Father! Father!" she screamed, gesturing toward the motionless body of a man lying nearby.

One man held the limp body of what appeared to be a girl or young woman. "Muslims, look at this," he cried.

In the West Bank, Palestinian President Mahmoud Abbas condemned the "bloody massacre" in Gaza and called on the international community, including the U.S., Europe and the Security Council, to intervene.

In Gaza, Prime Minister Ismail Haniyeh, a top Hamas official, condemned the attack. "What is going on is war crimes in the full meaning of the word," he said after visiting the hospital where the wounded were treated. He called for an end in recent Palestinian infighting.

Maj. Gen. Yoav Galant, Israel's southern commander, said an investigation was underway, but it appeared "uninvolved persons" had been harmed. He said the army was exploring whether an errant tank shell had hit the beach. "If indeed this is the situation, we have to see how to fix this," he said in a conference call with reporters.

"We take many security measures not to harm the uninvolved. Unfortunately, in a war operation, there are also mistakes. If indeed a mistake happened, we'll admit it," he said.

Lt. Gen. Dan Halutz, the army chief, ordered a halt in artillery fire on the area while the probe continued, the army said.

Israel frequently targets sites used by Palestinian militants to fire rockets toward Israel. In addition to the artillery fire, gunboats shelled targets and aircraft carried out three missile strikes. One airstrike killed three militants after they fired a rocket into Israel, Palestinian officials said.

The men were identified as members of the Popular Resistance Committees, a small group that is responsible for much of the rocket fire.

The group's leader, Jamal Abu Samhadana, was killed in a separate airstrike late Thursday. Abu Samhadana had recently served as commander of the Hamas government's private militia.

Tens of thousands of Palestinians, some firing in the air and calling for blood, flocked to a Gaza Strip stadium on Friday to bury Abu Samhadana, the highest-profile militant commander that Israel has killed in four years.

Hamas, sworn to Israel's destruction, condemned the attack. But it wasn't clear whether the group, which suspended its bombing war against Israel in February 2005, would take action against Israel directly or simply back other factions' operations, as it has done in the past.

Abu Samhadana was revered in Gaza as a key figure in Palestinian rocket attacks against Israel. A suspect in a deadly bombing of a U.S. convoy in Gaza in 2003, he maintained strong ties with the various Palestinian factions, and was a member of one of the most powerful clans in the teeming Rafah refugee camp where he lived.

Hundreds of gunmen escorted Abu Samhadana's body from the morgue to his house, and then through the streets of Rafah on the way to the stadium. They fired thousands of bullets in the air, chanting, "God is great" and "Revenge, revenge."

Rival militant factions, including gunmen affiliated with Abbas' Fatah Party, joined the procession, displaying rocket launchers, assault rifles and rocket-propelled grenades.

The prayer service proceeded quietly, and afterward, mourners streamed out of the stadium toward the cemetery where Abu Samhadana would be buried. Dozens of gunmen in the procession fired bullets in the air and some people chanted "God is great," and "We are ready to redeem you with our souls and our blood."

As a key Israeli target, Abu Samhadana had moved stealthily, switching cars and hideouts. Just a few days before his death, he told The Associated Press in a back alley interview that the U.S. government and its people would "pay a dear price" for leading bruising economic sanctions against the Hamas-run Palestinian Authority for its refusal to disarm militants and recognize Israel.

He said he had security measures in place against Israeli attack. "They don't catch me. I hunt them," he boasted.

Unidentified militants in the Gaza Strip fired four rockets into Israel on Friday, hitting a building in the southern town of Sderot, but causing no casualties, the military said. Israeli navy boats retaliated with artillery fire.

Abu Samhadana's appointment as Hamas' top enforcer helped to set the stage for recent Palestinian infighting that has killed 16 people and raised the specter of civil war between Hamas and the long-ruling Fatah movement it unseated in January parliamentary elections.

Abbas, a moderate who leads Fatah, is eager to restart long-stalled peace talks with Israel, and on Saturday, is to formally announce a July 31 date for a national referendum on establishing a Palestinian state alongside Israel.

Haniyeh on Friday sent a letter to Abbas, urging him not to hold the vote and to continue negotiations over the plan. He said the referendum would divide the Palestinian people and instead proposed forming a national unity government with Fatah.

"The idea of the referendum now on the table carries many dangers," Haniyeh wrote. "I'm afraid it will cause a history rift that will hurt the Palestinian cause for decades to come."

Opinion polls show the two-state proposal enjoys widespread support.