Italian national police in helicopters swooped down on a farmhouse near the Sicilian capital of Palermo early today and captured Michele Greco, known as the "boss of bosses," who is a central figure in the mass trial of 467 alleged Mafia criminals that opened there last week.

Police officials declared the arrest "one of the most important breaks in years" in their war against the Mafia, which U.S. and Italian crime fighters allege is the force behind the world's multibillion-dollar narcotics trade.

Known in Mafia circles as "the pope," Greco, 62, has been identified by a key informer and police investigators as the Mafia boss of Palermo, and as such the "chairman" of a 12-man Mafia "commission" said to rule the criminal syndicate.

The informer, ex-Mafia boss Tommaso Buscetta, broke the Mafia's traditional code of silence after two sons, a son-in-law, two brothers, two nephews and a score of friends were killed in a gang feud.

Buscetta is one of the key witnesses at the trial in a specially built courtroom on the grounds of Palermo's notorious Ucciardone prison.

Buscetta and another disgruntled Mafia figure, Salvatore Contorno, are being held in New York as witnesses against Mafia figures being tried in the so-called "pizza connection" case involving drug dealers who distributed heroin in the United States through a string of pizza parlors.

Michele Greco and his enforcer brother, Salvatore, "the senator," have been fugitives since 1982, when killings of public officials triggered the current Italian crackdown.

Both of the Grecos were sentenced in absentia to life imprisonment by an Italian court in 1984 on charges of having planned the car-bomb assassination of Palermo judge Rocco Chinnici, an opponent of the Mafia.

Both Grecos also are under indictment in the Palermo trial for drug trafficking; Mafia association; orchestration of half a dozen murders of Mafia rivals, judges and police, and the 1982 assassination of Palermo prefect general Carlo Alberto della Chiesa.

The murder of della Chiesa, who was credited with having broken the Red Brigades terrorist organization in the 1970s, set the stage for the current trial. It is being described as the most concerted crackdown on the Mafia since Mussolini ruthlessly curbed its power before World War II.

With a setting in the $19 million courthouse -- called "bunker hall" because of its seeming impregnability behind bulletproof glass and concentric circles of patrolled steel fences -- the trial opened Feb. 10 and has titillated the Italian public's love of drama and spectacle.

An 8,636-page indictment took a team of four constantly protected judges three years to prepare. One defendant alone was accused of moving about $600 million in drug profits through his Swiss bank account in two years. Its charges touch on 97 murders.

The indictment declares at the outset: "This is the trial of the Mafia organization called Cosa Nostra, a very dangerous criminal association, which with violence and intimidation has sown death and terror."

"I think this trial represents a great moment in the fight against the Mafia because for the first time all the facts, all the connections, all the activities of the Mafia are being viewed in their totality, not just piecemeal as in the past," said Ricardo Boccia, the high commissioner of the government's anti-Mafia task force, in an interview.

According to Boccia and other judicial officials, the single trial of 467 was necessary because they were all part of the same web of criminal activity.

"Although it is unwieldy and judicially difficult, we had to have this sort of maxi-trial to objectively present the complex connections between all elements in the criminal conspiracy," said prosecutor Domenico Signorino, one of the two men presenting the government's case. "If we had not done it this way, we never would have been able to expose the global picture of the problem."

Those with the lesser charges of "association" must be prosecuted within a year of the indictment -- that is, by Nov. 8, or go free. The more serious charges must be dispensed with by May 8, 1987.

At the courthouse, 220 defendants not on the lam, under house arrest or free on their own recognizance are housed in 30 steel cages that line five walls of the hexagonal building.

A roll call of defendants and their charges is required before each day's session. That has taken up to 2 1/2 hours daily, given interruptions caused by absences that have to be explained, apparently feigned epileptic attacks resulting in consultations with doctors, and the normal course of legal objections or debates.

Often the roll calls end just in time to break for the two- to three-hour Italian lunch.

Further delays already have been caused by the illness of jurors. Two have been excused for medical reasons that veteran Mafia watchers here suggest were triggered by Mafia intimidations of their families.

The court is made up of 2 judges, 6 regular jurors, and 10 alternates who attend daily and are on call to substitute. Should the juror supply ever drop below the mandatory six, a mistrial would be declared.

When a third juror called in sick early this week, the judge had to decide whether to continue the hearings or suspend the session in the hopes that the juror, complaining of a kidney stone, could return later in the week. Presiding Judge Alfonso Giordano chose to suspend the session until Friday.

"This is the third juror in six sessions to call in sick," said Marco Sassano, the veteran court reporter for the Milan daily Il Giorno. "There are hundreds of sessions ahead. If we continue this way we will be out of jurors before the real legal arguments and testimony even begin. Then the Mafia will have won again."

Dozens of Mafia trials have ended with no convictions, because witnesses suddenly forgot or reversed pretrial testimony or were killed, or judges and jurors were intimidated. CAPTION: Picture 1, Prisoners' cages in the $19 million, hexagonal courthouse in Palermo designed for mass Mafia trial. AP; Picture 2, Michele Greco . . . captured by copter-borne police; Map, Italy. The Washington Post