Italian police said today that they had thwarted an attack on the U.S. Embassy in crowded central Rome and have arrested seven men suspected of being Shiite terrorists.

Rome police chief Marcello Monarca said there was "irrefutable proof" that the seven men arrested Saturday, who are suspected of being members of the Islamic Jihad terrorist group, "were preparing a massacre."

He said maps and other documents found on the men and in the apartment they occupied in the nearby seaside resort of Ladispoli left no doubt that "they had a definite plan to carry out an attack on the American Embassy."

Speaking at a press conference this evening at Rome police headquarters, Monarca declined to confirm that the alleged terrorists' plan involved a suicide truck bombing against the four-story 19th century palazzo. The Islamic Jihad staged such raids against the U.S. Marine compound in Beirut October 1983 and a U.S. Embassy annex in a suburb of Beirut on Sept. 20.

"We are still examining the situation," he said, adding that the arrested men had maps of the embassy showing its entrances and its strong and weak points. The seven men have been charged with terrorist activities and attempted massacre by Italian prosecutor Domenico Sica, judicial sources said. U.S. Ambassador Maxwell Rabb thanked the Italian police tonight for "the speedy manner in which it had intervened in this case."

The ambassador's statement on the capture of the suspects said: "Their guilt or innocence will have to be established by the judiciary, but once again Italy has served as an example for the entire world of how to defeat the menace of terrorism."

[A State Department spokesman in Washington said: "We thank and applaud the Italian government and law enforcement authorities for the brilliant and painstaking investigative efforts which led to the uncovering of this possible plot against our embassy, and for the courage and dedication they have shown against terrorism."]

Monarca explained that news of the arrests was delayed until today because police were continuing their investigation.

He said the seven men, who had been under surveillance for some time, were picked up in Ladispoli on Saturday as a direct result of the arrest in Zurich on Nov. 18 of an eighth man, Hussein Hanih Atat, who allegedly was carrying more than four pounds of explosives. The man was reported to be about to board a plane for Rome, according to other Italian police sources.

The seven men arrested here, who are being detained in an undisclosed location near Rome, have been identified as Neemtalla Mohammed Fahs, 20; Hussein Adbul Hassan Safaoui, 22; Helhem Khodr Issa, 20; Mohammed Ramsi Arzouni, 21; Mahmoud Mohammed Gebara, 22; Mohammed Hani Bayoun, 22, and Mohammed Nabil Merhi, 23,.

According to police sources, the men, some of whom were carrying false passports, have refused to respond to questioning.

The U.S. Embassy, on Via Veneto, recently added heavy chains and cement blocks to its front entrance, which is generally guarded by special guards and some Italian police.

But there is no heavy wall around the embassy and visitors often can reach the front door without being questioned about their business.

Islamic Jihad is a name that has been given to a group of fundamentalist Shiite Moslems who may be linked to to Iranian ruler Ayatollah Ruhollah Khomeini, although some observers believe the group's name, which means "holy war," has been used by various Moslem terrorist groups.

The group has claimed responsibility for the April 1983 bombing of the U.S. Embassy in west Beirut, the Oct. 23, 1983, bombings of the U.S. Marine compound and a French paratroopers' barracks; the Dec. 12, 1983, attack on the U.S. Embassy in Kuwait, and the attack this September on the new U.S. Embassy annex near Beirut.

More than 360 people died in these attacks.