An investigation into an aborted, year-old Libyan plot to assassinate the U.S. ambassador to Rome is unearthing evidence that the Libyan embassy here was an important link in a broader, Libyan-supported terror network in both Italy and Egypt, with possible ties elsewhere in Western Europe.

Judicial sources said that the investigation has yielded the first tangible evidence here of a pattern of support by the Libyan People's Bureau, or embassy, in Rome for terrorism by furnishing weapons and money to Libyan agents to facilitate violence in Italy and other countries.

"While the information is still sketchy and far from conclusive," said an Italian source close to the investigation, "we think we may just be looking at the tip of the iceberg of the Libyan government's support network for terrorist operations in Europe."

The investigation surfaced earlier in the week when authorities arrested a former Libyan diplomat here and issued an arrest warrant for a Libyan diplomat who had already left the country.

The evidence obtained by Italian investigating magistrate Rosario Priore and prosecutor Domenico Sica, two of Italy's leading antiterrorist investigators, came as the Italian government today joined other European Community nations in expelling Libyan diplomats because of their government's apparent support for terrorism.

In a note issued today, the Foreign Ministry said it was ousting 10 of the 40 to 50 Libyan diplomats in Rome, Milan and the Sicilian capital of Palermo.

A ministry spokesman said the movements of all remaining Libyan diplomatic personnel would be limited to the province of the cities in which they work, unless prior government permission is obtained.

Italy also will not allow entry of any Libyan already banned from another European Community country because of links, or suspected links, to terrorism. Diplomatic pouches, suspected of being vehicles for the illegal entry of arms and explosives into Italy, will be rigidly controlled.

The governments of Britain, West Germany, Denmark, France, Belgium, the Netherlands, Luxembourg and Spain already have taken similar actions in accordance with the detailed decisions reached last Monday by foreign ministers of the 12 community countries. The Socialist government of Greece announced it would take no such action, saying evidence of Libyan involvement in terrorism is lacking.

Libya told ambassadors from eight West European nations in Tripoli Saturday that it would take unspecified reciprocal action, Reuter reported.

What made the Italian decision particularly significant was the direct link between the Libyan People's Bureau, or embassy, in Rome -- or at least some of its diplomats -- and specific terrorist plots as charted in the preliminary investigations by Priore and Sica.

Italy's interest in the Rome People's Bureau became evident earlier in the week when it was revealed that Italian police had arrested Arebi Mohammed Fituri, 47, an official in Tripoli's Libyan Arab Foreign Investments Co. here, who until the end of 1985 was a diplomat in the Rome People's Bureau.

Fituri has been charged with smuggling a gun into Italy. But judicial sources here said they suspect that it was Fituri, together with a second diplomat, who recruited another Libyan, 40-year-old Rageb Hammouda Daghdugh, to assassinate U.S. Ambassador Maxwell Rabb and the envoys from Egypt and Saudi Arabia.

The second diplomat, Mussbah Mahmud Werfalli, who Italian police believe is a more important member of the Libyan terror network than Fituri, was expelled from Italy last year.

Daghdugh was arrested on Feb. 5, 1985, in possession of a Walther P38 pistol and checks, variously reported to total up to $25,000, issued to him by the Libyan People's Bureau. Daghdugh, who is cooperating with the Italians, said that the pistol and the checks were given to him by Fituri and Werfalli for his participation in the plot to kill the three ambassadors.

Doubt was cast on Daghdugh's credibility after he hinted in a magazine interview published this week that he originally had been recruited by the Central Intelligence Agency to act as an informer for them, possibly even as an agent provocateur. Italian judicial officials revealed, however, that they had received other evidence incriminating Fituri and Werfalli that has reinforced Daghdugh's accusations.

The most intriguing new evidence, sources said, came from Egyptian authorities, who linked Werfalli with a Libyan plot to assassinate former Libyan prime minister Abdel Hamid Bakoush in Alexandria, Egypt, last November.

Four Libyans arrested by the Egyptians late last year in connection with the plot were sentenced to life imprisonment by an Egyptian court on Feb. 4. According to the sources, Egyptian authorities have now forwarded to Italy reports on their investigation of the four jailed Libyans, which name Werfalli and Fituri as Libyan secret agents engaged in the support of terrorism, as well as other diplomats in people's bureaus elsewhere in Western Europe who were similarly employed.

The view that this week's arrests merely scratch the surface of a Libyan terrorist network was reinforced by the finding of numerous blank Italian and Moroccan passports in a safe house this week.

Officials believe the house was frequented by Fituri after he returned to Rome this year to take up his nondiplomatic post with the Libyan investment company.