Italian police have arrested a former Libyan diplomat here in connection with an alleged Libyan plot to assassinate the U.S. ambassador and two other senior envoys a year ago, Public Prosecutor Domenico Sica said today.

After a long investigation, Sica said, police Sunday night arrested Arebi Mohammed Fituri, 47, a former Libyan diplomat.

They also issued an arrest warrant for a second former Libyan diplomat, Mussbah Mahmud Werfalli, 39, whom police believe to be living in Malta.

The government used the arrest and the issuance of a warrant to show a tougher Italian resolve against Libyan diplomats who have been implicated in instigating or supporting a recent wave of terrorism in Western Europe.

"This is a concrete sign of the Italian government's determination to fight against terrorism," Antonio Ghirelli, press secretary to Prime Minister Bettino Craxi, said tonight.

Government officials said further measures would most likely be taken against Libyan diplomats in Italy following a decision by the 12 European Community foreign ministers today to reduce the number of Libyan diplomats in Europe and to restrict their movement.

The two former Libyan diplomats named by prosecutor Sica have been charged with possession of a .38 caliber pistol, which, court sources allege, they gave a year ago to a third Libyan, Rageb Hammouda Daghugh. The weapon was to be used in attacks allegedly planned against U.S. Ambassador Maxwell Rabb, and the Egyptian and Saudi Arabian ambassadors to Rome.

Daghugh was arrested and jailed in 1985 for possession of a .38 caliber pistol with a silencer, according to court sources here. He also had a check for $2,000 from the Libyan People's Bureau, or embassy.

At the time, both Fituri and Werfalli, who were protected by diplomatic immunity, were ordered expelled from Italy because of their connection to Daghugh.

Werfalli, Italian police believe, has remained aboard since then. But Filturi, they believe, returned recently as an employe of the Libyan government's investment holding company, Libyan Arab Foreign Investment Co. Among the company's holdings is a 15 percent share of the Italian automotive and industrial firm, Fiat, S.p.a.

Daghugh, who police believe was the chosen hit man for the planned attacks, was jailed for a year. He was released on provisional liberty when the legal time limit for him to be held without a trial expired. His current whereabouts are unknown.