The ruling Hamas group fired a barrage of homemade rockets at Israel on Saturday, hours after calling off a truce with Israel in anger over an artillery attack that killed seven civilians in Gaza.

The Hamas militants' announcement to call off the cease-fire, reached in February 2005, raised the prospect of a renewed campaign of deadly suicide bombings and a wave of bloodshed.

Despite Israel's attacks and Hamas pressure on President Mahmoud Abbas to hold off on a planned referendum on establishing a Palestinian state alongside Israel, the Palestinian leader was expected to announce later Saturday a decree to hold the vote on July 31.

In the hours before the announcement, new violence erupted between gunmen loyal to Abbas and Hamas' militia.

CountryWatch: Israel

Hamas also claimed responsibility Saturday for firing at Israel at least 15 rockets, as well as a barrage of mortar bombs. The attacks caused no casualties.

"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."

Israel's artillery attack on Friday was part of a wider aerial and artillery bombardment of suspected Palestinian rocket-launching sites that killed a total of 10 people, three of them militants.

The violence fueled tensions already high over an Israeli airstrike on Thursday that killed the Hamas government's top militant commander, Jamal Abu Samhadana.

Friday's artillery attack tore into a beach-side picnic in Gaza, killing seven civilians, and leading Abbas to condemn the attack 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.

But overnight Saturday, gunmen shot and killed a 39-year-old Palestinian security officer in a botched kidnapping attempt, the Palestinian Preventive Security said. The force, loyal to Abbas and Fatah, accused Hamas of the killing and vowed to "stop Hamas' game whatever the price."

During the funeral procession for Maj. Bassam Qutub, a gunbattle erupted between gunmen from Hamas and the Preventive Security force. The car of Gaza's top general, Rashid Abu Shbak, was caught in the crossfire, and he had to be rushed out of the area while under fire.

Four people were wounded in the fighting, one of them seriously, hospital officials said. Hamas gunmen were brought into the area to take up position, leading to a tense standoff with the Preventive Security officers.

Furious over Qutub's killing, Fatah boycotted on Saturday a meeting with Haniyeh. The meeting was aimed at calming the tensions between the groups, but Fatah spokesman, Tawfiq Abu Khoussa, said Hamas had committed a "number of violations" in recent days, including killing Qutub.

In Beit Lahiya, thousands of Palestinians gathered to attend the funeral procession of the seven people killed by Israel on Friday.

Chanting "revenge, revenge" and "destroy Israel, destroy America," the mourners made their way through the town's narrow streets to the families' homes, where weeping women tried to touch the bodies of the dead.

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

"Harming innocent civilians is of course totally unacceptable and we will do whatever we can to avoid civilian loss of life," said Israeli Foreign Ministry spokesman Mark Regev.

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.

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. Ghobn 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.

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

Hamas, which is sworn to Israel's destruction, wants changes in the language of the proposal. Abbas has refused to comply with Hamas' demands, and public opinion polls show the two-state proposal enjoys widespread support.