PARIS (AP) _ A special court ignored a prosecution plea for leniency and sentenced a Lebanese terrorist to life in prison Saturday for complicity in the shooting deaths of two diplomats and the attempted killing of a third.

Two of the victims were Americans.

Paris shuddered under a wave of bombings last year as terrorists tried to force the government to free the defendant, Georges Ibrahim Abdallah. The prosecution asked for leniency in an effort to spare France new terrorism.

Defense lawyer Jacques Verges said the sentence, the maximum possible, would look to ''many Arab militants like a declaration of war.''

''Indeed, we can expect mass or individual attacks,'' said Georges Kiejman, a lawyer representing the United States in the case. He called the sentence ''very courageous.''

The government quickly ordered that the heightened security in Paris during the trial be maintained, and called in army troops to reinforce security on the frontiers, the news agency Agence France-Presse reported.

Interior Minister Charles Pasqua, in charge of domestic security, canceled a visit to Gabon due to start Sunday, the ministry announced.

Abdallah refused to attend the weeklong trial and was not present when the verdict and sentence were announced. Verges said the 35-year-old Abdallah laughed upon hearing the sentence and told him he would not appeal.

Prosecutor Pierre Baechlin asked the seven-judge court to sentence Abdallah to no more than 10 years, saying a harsher sentence would ''transform the accused into a martyr and France into a hostage.''

Police described Abdallah as a leading member of a Lebanese terrorist group. His arrest in October 1984 touched off two bombing waves in Paris, including one last September in which 11 people were killed and dozens injured. Police have said Abdallah's four brothers are the prime suspects.

Abdallah was convicted of complicity in the Jan. 18, 1982 murder of Lt. Col. Charles Ray, a U.S. Embassy deputy military attache, and the April 3, 1982, murder of Israeli diplomat Yacov Barsimantov. He also was convicted in the attempted murder of Robert O. Homme, then U.S. consul general in Strasbourg, on March 26, 1984.

The Lebanese Armed Revolutionary Faction claimed responsibility for all three attacks. French authorities said Abdallah belonged to the group. Abdallah described himself as an ''Arab fighter.''

The court, created to handle terrorism cases, deliberated for 70 minutes. A loud gasp came from many of the approximately 150 spectators and journalists in the heavily guarded courtroom when presiding Judge Maurice Colomb announced the verdict and sentence.

Colomb said the court reached a majority decision that Abdallah was guilty on all three counts without extenuating circumstances.

Abdallah must serve at least 15 years before becoming eligible for parole. He now is serving a four-year term, imposed in July, for criminal association and possession of arms, explosives and false documents.

Verges later said Abdallah ''considers himself at the end of the trial the winner of this battle. I am smiling, he is laughing.''

''I think that to the extent this verdict was based on an empty dossier ... it cannot but appear to many Arab militants as a declaration of war,'' Verges said. ''I don't think that one can keep a man like Georges Ibrahim Abdallah without it having some significance ... for his friends.''

Asked about the possibility of renewed terrorist attacks in France, the U.S. ambassador to France, Joe M. Rodgers, said: ''We will have to take the consequences of defending democracy as we have in our world wars and in other ways. We're at war, and we're defending democracy.''

On Friday, Baechlin said in his closing arguments that security forces and the justice system are ineffective against terrorism. He urged the judges to be lenient and ''adapt to events.''

Baechlin told reporters Saturday, before the verdict was announced, that the government had not ordered him to ask for leniency.

''But I am a senior official, able to gather information. That is what I did in the past 48 hours, and that is what guided my conduct. Friday morning I had access to information ... and I assumed my responsibility,' ' he said without elaborating.

Kiejman asked in his closing arguments for a stiff sentence, saying France should not be made into a sanctuary for terrorism.

After the September bombings, it was widely reported the government made a deal with the Lebanese Armed Revolutionary Faction to stop the attacks in return for a speedy trial and light sentence for Abdallah.

The United States protested what it said was the leniency of the previous Abdallah verdict in July and became a civil party in this case.

The court also ordered Abdallah to pay $24,600 to Sharon Ray, widow of the slain U.S. diplomat, and $16,400 each to her two children. It awarded symbolic damages of one franc each to the United States and to Homme.

Mrs. Ray, interviewed by telephone from the United States, told French television: ''I have not slept for five years, and this makes me feel at peace.''

She told the station Antenne Deux she was ''very pleased at the justice system,'' and praised the judges ''for their courage, especially their courage in the sentence that they provided.'' Antenne Deux did not say where in the United States it contacted Mrs. Ray.