Palestinian gunmen waiting in ambush opened fire and threw grenades at a group of Jewish settlers walking home Friday evening after marking the start of the Sabbath at a shrine in the volatile West Bank city of Hebron, killing at least 12 of the Israelis and wounding 15, Israeli military officials said.

The Jewish worshipers, accompanied by officers from Israel's Border Police, were making their ritual walk to the nearby settlement of Kiryat Arba after praying at the Tomb of the Patriarchs, a site revered by Jews and Muslims, when they were attacked about 7:10 p.m. by an unknown number of Palestinians in what appeared to be a well-planned assault, an Israeli military spokesman said. He said the group of settlers came under "very accurate sniper fire" from a house along the route.

When Israeli soldiers and ambulances rushed to the scene to rescue the group, the spokesman said, they also were ambushed. Israeli television reported that gunfire continued in the city late into the night. Wounded Israelis were still being evacuated from the scene four hours after the incident began.

Sporadic but intense gunfights broke out, and Israeli radio said at least two Palestinians were killed. Israeli military helicopters thumped over the city and flares lighted the night as gunfire cracked intermittently.

Israeli military officials said the dead Israelis included civilians, soldiers and members of the Border Police. Officials said the dead were about evenly divided between civilians and security forces.

The Israeli casualties, they added, included a colonel who was the senior army commander in the ancient city, which lies about 15 miles south of Jerusalem and is uncomfortably divided between 130,000 Palestinians and 450 Jewish settlers. Over the years it has been the scene of some of the most incendiary religious violence in the West Bank.

"It was a premeditated ambush to murder Jews," said Raanan Gissin, a spokesman for Prime Minister Ariel Sharon. "The response will be with the same measure."

The attack, which Israeli officials and media called the "Sabbath Massacre," was the bloodiest in Hebron since 1994, when a Jewish settler from New York, Baruch Goldstein, stormed into a mosque at the Tomb of the Patriarchs and gunned down 29 Muslim worshipers. Goldstein was beaten to death by survivors.

The leader of Islamic Jihad, Ramadan Shallah, asserted responsibility for the attack in a telephone interview with al-Jazeera satellite television, saying his radical Palestinian group carried it out in retaliation for Israel's killing of one of the group's military leaders, Iyad Sawalha, a week ago in the northern West Bank town of Jenin.

The violence came 10 days after the collapse of Israel's coalition government and at the start of what is expected to be a tense, 10-week election campaign focused on demands for tougher security. More than 600 Israelis and 1,800 Palestinians have been killed in the two-year-plus Palestinian uprising over continuing Israeli occupation of the West Bank and the Gaza Strip. Sharon, who is running for reelection in the right-wing Likud Party, likely will face stiff pressure to respond.

"It's a very sensitive political season, and the domestic situation is very fluid because the candidates of both major parties have yet to be selected, and an enormous number of people report that they have yet to make up their minds," said Asher Arian, a senior fellow at the Israel Democracy Institute in Jerusalem. "But I think what we know from past elections is that incidents like this add votes to the right wing, and of course the closer to the election, the more likely that happens."

Early today, Israeli soldiers were surrounding the house used by the snipers during the initial attack, an Israeli military spokesman said. He said they believed the gunmen were still inside.

The assault came as the worshipers were returning to Kiryat Arba, a settlement with about 6,000 residents a little more than a half-mile east of Hebron's old city and the Tomb of the Patriarchs. The settlers were about halfway home on a winding village road when they were ambushed, Israeli officials reported.

"This was very well planned, clearly, because the path taken by these people was repeated every Friday night, and the time was predictable," said an army spokesman, Capt. Jacob Dallal. "It's not as if the Palestinians were ambushing an army patrol. They ambushed civilians and the troops who came to rescue them."

Hebron's a bloody history is a product of years of struggle between competing religious groups over rights to the holy site revered by Jews, Muslims and Christians and exacerbated in recent decades by conflicts between Jewish settlers living in the midst of the city and its Palestinian majority. Both groups have become radicalized, and tensions between Jews and Arabs in Hebron are among the most vicious in the West Bank.

About 450 settlers live in four enclaves in the city, protected by Israeli military forces. Although the Palestinian Authority controls most of Hebron, about 30,000 Palestinians living near the Jewish settlers are under the control of the Israeli military, which frequently imposes curfews and other restrictions.

In 1929, 67 Jews were killed by Arabs during rioting that prompted British authorities in what was then Palestine to evacuate Jews from the city. But when Israel captured the West Bank from Jordan in the 1967 Middle East war, setters began returning to what had been Hebron's Jewish quarter. The large, rambling settlement of Kiryat Arba was established in 1971 on the eastern side of the city.

According to religious teachings, Abraham purchased caves that now lie beneath a shrine on the site as the tomb for his wife. Abraham, his wife, his sons and most of their wives were believed to have eventually been entombed in the caverns. Over the years, as Jews and Muslims took turns conquering the area, both a mosque and a synagogue were built at the shrine. Today a large multi-level building sits at the site with separate worship areas for Muslims and Jews.

An injured Israeli settler is put into an ambulance at the Gush Ezion junction near Hebron. The wounded were still being evacuated hours after the attack. An injured Israeli settler is moved at a Jerusalem hospital. Israeli forces were watching a house where they suspected the gunmen might still be hiding.