An Israeli court, climaxing a 13-month trial of members of a Jewish underground terrorist organization that operated in the Israeli-occupied West Bank for four years, today found 15 men guilty of crimes ranging from murder and attempted murder to illegal possession of weapons and conspiracy.

Three of the defendants, including Menachem Livni, who was described in court as the mastermind behind the organization's attacks on Arab targets, were convicted of murder and under Israeli law face mandatory sentences of life imprisonment. They and the other 12 defendants, whose maximum punishments could range up to 20 years in prison, are to be sentenced next week.

Today's verdicts by a sometimes divided three-judge panel ended the most spectacular criminal case in Israel's history, pitting the country's legal system against the most militant of the Jewish settlers who live in the West Bank.

The major crimes involved in the trial included the 1980 attempt to assassinate three Palestinian West Bank mayors, the 1983 murder of three Arab students at the Islamic College in Hebron, the 1984 planting of bombs on five civilian Arab buses in East Jerusalem and an unsuccessful conspiracy to blow up the Dome of the Rock Mosque on the Temple Mount, a site revered by both Jews and Moslems in Jerusalem's Old City.

Throughout the trial, which began in June 1984, the orthodox Jewish defendants portrayed themselves as defenders of Jewish rights in the West Bank who acted in response to acts of terrorism by Arabs. Most signed confessions of the crimes they were accused of, but later unsuccessfully sought to prevent these from being introduced as evidence in court.

The guilty verdicts and possibility of long prison sentences did not appear to shake the defendants' belief in the righteousness of their cause. One of them, Yitzhak Novik, 37, who was convicted of placing a bomb in June 1980 that crippled the late Karim Khalaf, then mayor of Ramallah, told Israeli radio outside the courtroom:

"I am personally disappointed by the decision categorizing me as a terrorist, especially since what I did I felt I did in order to protect my family and my neighbors. Looking back, it's been proven that what I set out to do was successful in that for two years after what I did there were almost no grave terrorist incidents in the West Bank."

Another defendant, Yehuda Etzion, 34, was convicted of attacking the Arab mayors and of conspiring to blow up the Dome of the Rock, considered the third holiest site in Islam.

"I'm not surprised," he said. "I knew it was going to be. But I think that in the court of history I'm 100 percent not guilty because the building will be removed from the Temple."

The suspects were strongly defended throughout the trial by the leaders of the Jewish settlement movement in the West Bank and by some members of Israel's parliament from the Likud bloc and other right-wing parties. Their convictions today are expected to lead to calls from the settlement activists for the pardoning of the defendants by Israeli President Chaim Herzog.

In May, when Israel released 1,150 Arab prisoners in exchange for three captured Israeli soldiers, there were attempts by some here to link this decision to the trial of the Jewish underground. They argued that the Jewish suspects should be freed immediately because Israel had released so many Arab prisoners, many of them convicted terrorists.

Herzog said at the time that he would consider no appeals for pardon until after the trial, and then only on a case-by-case basis.

In all, 27 men, most of them residents of West Bank settlements, were arrested and charged in connection with the Jewish underground activities. Ten of these were convicted earlier under plea bargaining agreements. Two other suspects, both Israeli Army officers charged with conspiracy in connection with the attack on the Arab mayors, are to be tried separately after the other defendants are sentenced.

There are also two other men being sought in connection with the case, one of whom, Ira Rapaport, fled to the United States.

Several of the defendants have distinguished military records from Israel's wars. Invariably described by friends and supporters as devoted family men, what they most share in common is a commitment to Israel's retention of the West Bank as a God-given right of the Jewish people.

Livni, 38, the leader of the underground organization, is an engineering corps battalion commander in the Army reserves and was in the forefront of Jewish settlers who moved into the Arab city of Hebron after Israel captured the West Bank in the war of 1967.

At the outset of the trial, the defendants announced they intended to use it to dramatize their charge that the Israeli government was lax in providing security for Jewish settlers in the West Bank, forcing the settlers to take the law into their own hands. But when this line of defense was ruled inadmissible in the trial, 13 of the 15 defendants refused to testify, saying they had been deprived of the chance to explain their actions.

Judges Zvi Cohen and Shmuel Finkelman, in a majority opinion for the three-judge panel, said this suggested that the defendants had no defense for their actions.

"If there is no other line of defense besides the motive behind the acts, there is no line of defense and there is no defense," the judges wrote.

According to evidence presented in court, the terrorist organization in which most of the defendants were active, was formed in the late 1970s around the idea of destroying the Dome of the Rock Mosque. This was never carried out and the court split 2 to 1, with Cohen and Finkelman ruling that a conspiracy to blow up the Moslem holy place had taken place, while Chief Judge Yaakov Bazak ruled there had been no conspiracy.

On June 2, 1980, according to today's verdict, the underground struck for the first time in a crime that deeply disturbed the Palestinian residents of the West Bank. Its targets were five prominent members of the Palestine National Guidance Committee, the Arab leadership organization in the occupied territory.

Mayor Khalaf of Ramallah was crippled and Bassam Shaka, then the mayor of Nablus, lost both legs to bombs placed in their cars. An Israeli Druze bomb disposal expert was blinded while attempting to defuse a third bomb, at the home of Ibrahim Tawil, then the mayor of El Bira. Attempts to plant bombs near the other two targets failed.

The failure of Israeli authorities to make arrests in this case for four years was a source of bitter complaint among West Bank Arabs. In today's verdicts, charges of attempted murder in the case were disallowed by the court, but nine of the defendants were convicted of causing grievous harm, a crime that carries a maximum sentence of 20 years, the same as for attempted murder.

The daylight assault on the Islamic college in Hebron occurred on July 26, 1983, when two men with automatic rifles and hand grenades charged into a group of students. Three students were killed and more than 30 wounded.

Livni, Shaul Nir, 31, and Uzi Sharabaf, 25, were convicted of murder and attempted murder in the assault and two other defendants were convicted of manslaughter and attempted manslaughter. Sharabaf is the son-in-law of Moshe Levinger, a prominent West Bank rabbi and leader of the Jewish settlers in Hebron.

The last major crime of the underground that led to the arrests was the planting of bombs on five civilian buses at the main East Jerusalem bus terminal on April 28, 1984. The bombs were discovered and defused before they exploded. Four of the defendants were convicted of attempted murder in that case.

Other charges on which many of the defendants were convicted included illegal possession of arms and explosives, damage to military property and membership in a terrorist organization.