Hamas militants called off a truce with Israel on Friday after a barrage of Israeli artillery shells tore into Palestinians at a beachside picnic in the Gaza Strip, killing seven civilians.

Palestinian Prime Minister Ismail Haniyeh, from the Islamic group Hamas, visits a man wounded during an Israeli strike in Gaza City June 9, 2006. One artillery barrage fired by Israeli forces killed seven Palestinian civilians, including three children, at a family picnic at the beach Friday. The violence included a missile strike that killed three Palestinian militants. The Israeli army said it had targeted areas in the northern Gaza Strip used by Palestinian militants to fire homemade rockets at Israel. After the incident, Hamas' military wing said it would no longer honor the truce with Israel.

The declaration raised the prospect of a new wave of bloodshed. Hamas militants suspended a campaign of deadly suicide attacks on Israelis with a February 2005 cease-fire, and have largely stuck to the truce. The Islamic group now leads the Palestinian government.

"The earthquake in the Zionist towns will start again and the aggressors will have no choice but to prepare their coffins or their luggage," the Hamas militants said in a leaflet. "The resistance groups ... will choose the proper place and time for the tough, strong and unique response."

The Israeli artillery attack was part of a wider aerial and artillery bombardment of suspected Palestinian rocket-launching sites that killed a total of 10 people Friday.

The violence fueled tensions already high over an Israeli airstrike that killed a militant commander in the Hamas-led government Thursday.

Tens of thousands of people packed a Gaza soccer stadium Friday for Jamal Abu Samhadana's funeral. They fired thousands of bullets in the air, chanting, "God is great" and "Revenge, revenge."

Palestinian President Mahmoud Abbas condemned the attack on the beach as a "genocidal crime." He called for international intervention and declared a three-day period of mourning. His rival, Prime Minister Ismail Haniyeh of Hamas, said the shelling was a "war crime" and urged an end to recent fighting between Hamas and Abbas' moderate Fatah movement.

The killings raised questions about whether Abbas would go ahead with referendum on establishing a Palestinian state alongside Israel despite pleas from Hamas to hold off.

Abbas is eager to restart stalled peace talks with Israel, and on Saturday was expected to formally announce a July 31 date for the referendum. An official close to Abbas said late Friday the president would make his announcement as planned on Saturday.

The Israeli army said its attacks were aimed at areas that Palestinian militants used to fire homemade rockets at Israel.

Maj. Gen. Yoav Galant, Israel's southern commander, said investigators were trying to determine if an errant tank shell caused the bloodshed at the beach.

"I express deep regret over the fact that uninvolved persons have been hit," Galant told reporters. "We shall try to find a way to ensure not to harm the uninvolved."

Kamal Ghobn said he had just arrived at the beach on a bus with about 50 relatives when the attack took place. "I was still parking the bus and everyone got out to go to the beach. As I locked the door I felt the thud of the shells and felt a sting in my side," said Ghobn, who was slightly wounded by shrapnel. Gobn said he saw four shells land.

The artillery fire scattered body parts, destroyed a tent and sent bloody sheets flying into the air. A panicked crowd quickly gathered, screaming and running around hysterically.

A sobbing girl lay in the sand, crying for her father. "Father! Father!" she screamed.

The body of a man lay motionless in the sand nearby.

Palestinian officials said seven people were killed and more than 30 wounded at the beach. Hardest hit was the Ghalia family, which lost six members, among them the father, one of his two wives, an infant boy and an 18-month-old girl.

"This was his first day at the beach this summer. He was taking his kids to play. It's destiny," said Nasreen Ghalia, a sister-in-law of the dead father. She said one of the survivors was a 7-year-old girl, Hadeel, who had not been told she had lost her parents and siblings.

"Hadeel is now an orphan," she said.

In an Israeli airstrike elsewhere in northern Gaza, three militants were killed after they fired a rocket into Israel.

Hamas staged a series of large demonstrations across the Gaza Strip late Friday. In Gaza City, leaders quoted from the militant wing's statement calling off the cease-fire with Israel.

"We cannot remain silent," said Hamas spokesman Sami Abu Zuhri. "God willing, the reprisal is going to be earth shaking. We have no option but to defend ourselves, our people, our children and our land."

France called the Israeli shelling of civilians "disproportionate" and urged calm in the region.

The tough stance by Hamas was likely to deepen a dispute between the group and Abbas.

Haniyeh sent a letter to Abbas on Friday 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 unity government with Fatah.

But late Friday, Palestinian lawmaker Saeb Erekat, a confidant of Abbas, said the president planned to announce the referendum on Saturday as planned.

Public opinion polls show the two-state proposal enjoys widespread support.

But an angry backlash after Friday's killings could erode Abbas' support. Crowds chanted anti-referendum slogans during Friday's Hamas demonstrations.

Abbas has endorsed the referendum plan as a way to end international sanctions against the Palestinians and restart peace talks.

Hamas, which is sworn to Israel's destruction, wants changes in the language of the proposal, and the latest violence is likely to only deepen the group's hard line.