PARIS, FEB. 28 -- A special seven-judge tribunal, rejecting the prosecutor's plea for clemency, today convicted a Lebanese radical of complicity in the terrorist assassinations of a U.S. and an Israeli diplomat in Paris and sentenced him to life in prison.

The severity of the sentence was a surprise after yesterday's strained appeal by prosecutor Pierre Baechlin for a light penalty as a way to avoid further terrorist attacks in France and encourage liberation of French hostages in Lebanon.

French lawyers said it means the alleged terrorist leader, Georges Ibrahim Abdallah, 35, could spend 15 to 20 years in prison before parole, unless he receives a pardon from President Francois Mitterrand.

Abdallah's defense lawyer, Jacques Verges, warned that the guilty verdict and life sentence could be seen as "a declaration of war" by some militant Arabs. His comment was interpreted as a reference to threats French authorities have reported receiving in connection with the trial. A wave of bombings in Paris last fall by a group demanding Abdallah's release killed 11 persons.

Verges, his voice rising, said Abdallah will not appeal the conviction or sentence. Abdallah, who refused to attend the trial after its opening session and who was not present in the courtroom today, was notified of the verdict immediately and laughed at the news, Verges added.

"France will have to keep Georges Ibrahim Abdallah forever, if this is in the interest of the nation," Verges declared.

Black robes flowing, Verges then walked out of the ornate courtroom. He refused to participate in court proceedings on a parallel U.S. government civil suit for damages to be paid by Abdallah, referring to the hearing as "this derisory debate."

U.S. Ambassador Joe Rodgers, in a news conference later, said the Reagan administration was pleased at the outcome. The U.S. government participated in the trial and, through diplomatic statements and contacts, urged the French government to prosecute Abdallah to the full extent of the law.

"What we sought in this trial was justice, and justice has been rendered," Rodgers said, adding later: "In defense of democracy, we need to take a stand, and I am just very proud of the French government for taking a stand in defense of democracy."

The special tribunal convicted Abdallah of complicity in the 1982 assassinations in Paris of Lt. Col. Charles Ray, identified by the U.S. Embassy as a deputy military attache, and Yaacov Barsimantov, identified by the Israeli Embassy as a second secretary. He also was convicted of complicity in the 1984 attempted assassination of U.S. Consul Robert Homme in Strasbourg.

In the civil suit, the tribunal ordered Abdallah to pay one franc in symbolic damages to the United States and another franc to Homme. In addition, it ordered the Lebanese revolutionary to pay 150,000 francs, or about $24,600, to Ray's widow, Sharon, and 100,000 francs, or about $16,400, to each of their minor children. There was no indication where the money would come from.

The severity of the sentence contrasted markedly with Baechlin's plea for a maximum of 10 years. His unusually outspoken appeal, which he said was made "with a heavy heart," seemed to confirm widespread speculation that the government of Prime Minister Jacques Chirac was hoping for a light sentence that would make Abdallah's release possible later this year.

Abdallah has been in French jails since surrendering at Lyons in October 1984. He was sentenced to four years for possession of illegal arms and false documents. A concurrent 10-year sentence, with four years suspended, would have made him eligible for parole by next October, halfway into the six years, French newspapers pointed out.

Georges Keijman, a prominent French lawyer who represented Sharon Ray and the United States, said Baechlin was acting "in the name of the government" when he sought the light sentence as a means to avoid further terrorism in Paris.

"But in the final analysis, it was up to the judges to decide," he added.

The special antiterrorism tribunal was formed last month after civilian jurors in another terrorism trial refused to serve. Although they invoked a variety of excuses, the jurors were thought to be heeding warnings of retribution from the defendant.

Because of the terrorism threats surrounding Abdallah's trial, the seven judges received close personal protection and 1,000 extra security agents were assigned to Paris, French officials said.

Interior Ministry officials told French reporters the extra agents would be maintained, leaving a total of about 3,500 on duty in the Paris area, and that Interior Minister Charles Pasqua postponed a scheduled trip to Africa to monitor the situation.

Keijman said he does not disregard the dangers now faced by France because of what he called "this courageous decision. We may be the object of individual or collective attacks," he acknowledged.

A shopping center in Bordeaux was evacuated several hours after the sentencing because of a bomb threat telephoned to the French news agency, Agence France-Presse. The AFP bureau in Bordeaux reported the callers claimed they had planted a bomb in retaliation for Abdallah's sentence, but police found nothing suspicious.

French and U.S. officials have said Abdallah is a top leader of the Lebanese Armed Revolutionary Faction, a group that has acknowledged responsibility for eight attacks against U.S. or Israeli targets in Europe since 1981.

They alleged that Abdallah, from the Maronite Christian village of Qobayat in northern Lebanon, has been associated with radical Palestinian organizations, including that of the late terrorist chief Wadih Haddad.

Haddad's group kidnaped a French cultural attache in Lebanon in 1985 to bargain for Abdallah's release, French authorities charged. According to French and U.S. officials, a deal was arranged and the diplomat was freed, but France reneged on its end because new evidence was discovered linking Abdallah to the Ray and Barsimantov assassinations.