Israel has killed the leader of the radical Palestinian faction, the Popular Front for the Liberation of Palestine.

Abu Ali Mustafa, chief of the Popular Front for the Liberation of Palestine, was killed when missiles fired by Israeli combat helicopters slammed into his office in the West Bank town of Ramallah. The attack blew out windows and blackened the outside of the building. Inside, broken bloodstained furniture and shattered glass littered the floor.

The Popular Front for the Liberation of Palestine is a militant faction of the Palestine Liberation Organization that opposes peace talks with Israel.

Mr. Mustafa is the highest-ranking Palestinian official to be killed in a targeted Israeli attack since the current conflict began 11 months ago.

During the period, more than 50 Palestinians have been killed in operations by the Israeli military.

The Israeli army said in a statement that the Popular Front for the Liberation of Palestine is responsible for a series of attacks on Israelis, including an attempted car bombing in Jerusalem last week.

Israeli government spokesman, Raanan Gissen defended the decision to kill Mr. Mustafa. "We have a policy, which is actions of self-defense that we take when we see that there are terrorists who are planning, instigating, terrorist activity," he said. "He was not taken out because of just past events or past terrorist activity, but the fact that he was planning, while he was sitting in his office, additional car bombs."

The Palestinian Authority said in a statement that Israel "has opened the gates to an all-out war" by killing Mr. Mustafa.

A spokesman for Palestinian leader Yasser Arafat, Nabil Abu Rudeina, called Israel's policy of "assassinations, very dangerous." "This is a real murder," he said. "This assassination is a dangerous escalation and the situation will become definitely worse and worse."

Thousands of Palestinians marched in spontaneous protests across the West Bank and Gaza to protest the killing.

Mr. Arafat called for a three-day mourning period following Mr. Mustafa's death.

Mr. Mustafa replaced George Habash as the leader of the Popular Front for the Liberation of Palestine last year. He was the second in command of the group when it drew world attention to the Palestinian struggle by hijacking planes in the 1960s and 1970s.

Mr. Mustafa returned to the West Bank from exile in 1999, and was described as one of the top figures in the PLO.

Nearly 750 people have been killed, mostly Palestinians, since the uprising against Israeli occupation began last September.