Issam Sartawi was the most prominent of a number of Palestinian moderates to be murdered in recent years, and his assassination suggests that extremists may be turning more to terrorism at a time of divisions and difficulties for the Palestinian movement.

A radical Palestinian guerrilla group known by the name of its leader, Abu Nidal, claimed responsibility for the attack in Portugal.

"It is our pleasure to communicate to you our success in implementing the death sentence toward a criminal and a traitor," the faction's Revolutionary Council said in a statement released in Damascus, Syria.

Sartawi, the PLO's envoy to Western Europe, was the organization's only prominent leader publicly to urge explicit recognition of Israel's right to exist in exchange for U.S. recognition of the PLO. He helped set up important meetings between Arafat and Israelis sympathetic to the Palestinian cause, and he was a key contact between the PLO and sympathetic Western European leaders.

Abu Nidal's group has acknowledged killing other moderate Palestinians in Western Europe. The assassination of Sartawi, who once helped stage guerrilla attacks himself, apparently shows that Abu Nidal remains willing to use violence to intimidate rivals.

Arab officials who were involved in trying to bring PLO Chairman Yasser Arafat into an agreement for Jordan's King Hussein to join U.S.-sponsored peace talks said last week that Arafat's temporizing was due in part to a fear that he would be marked for assassination if he gave Hussein a mandate. The officials said that there had been direct threats to that effect from extremist groups in Damascus.

Syria, a leading Arab hard-liner toward Israel, has strongly opposed Arafat's flirtation with Hussein.

A Syrian-based extremist faction of the PLO, the Popular Front for the Liberation of Palestine-General Command, said yesterday, "Those who assassinated Egyptian president Anwar Sadat also assassinated Sartawi, and they will remain vigilant against anyone who tries to overstep the people's will."

Syria and Iraq have provided support for Abu Nidal even though he broke with the PLO in 1972 and was sentenced to death by the organization in 1978. He was reported last autumn to have transferred his base from Damascus to the Iraqi capital of Baghdad, although he reportedly continues to visit Syria.

Sartawi had survived three assassination attempts by Abu Nidal's group, Joseph Fitchett of the International Herald Tribune reported. The faction also claimed responsibility for shooting the Israeli ambassador to Britain June 3, the attack that triggered Israel's invasion of Lebanon three days later.

The PLO, which follows a line between Abu Nidal and Sartawi, had rejected Sartawi's calls to recognize Israel. Nevertheless, a PLO spokesman yesterday condemned the killing and blamed it on the Israeli secret service. Arafat, in North Yemen, was quoted by the national news agency there as calling the killing "a great loss to the Palestinian revolution."

The Israeli government also decried the assassination and denied that it was responsible. "One thing we see in the Arab world is that people who talk to the Jews or Israelis somehow are shot and killed," Cabinet Secretary Dan Meridor said.

Washington Post correspondent Jonathan C. Randal added the following from Rome:

Handsome, urbane, always elegantly turned out, Sartawi typified the westernized Palestinians who desperately sought dialogue with the Israeli fringe willing to envisage a Palestinian state on the West Bank.

Although tireless and outspoken, Sartawi never made much of a dent on his target audience or with fellow members of the PLO, which regularly disavowed his comments. In February, he was denied the right to address either the mainstream Fatah organization or the Palestine National Council, effectively the PLO's parliament-in-exile, in Algiers.

He also was suspect in Israeli eyes because of his participation in a bloody hijacking of an Israeli aircraft in the early 1970s.