Federal law enforcement officials, acting under a new extradition treaty with Italy, have begun to arrest 28 high-level organized crime figures sought by the Italian government in a crackdown on the Sicilian Mafia.

The 28, all Sicilian-born, are among 366 people named in Italian arrest warrants last weekend in an offensive against Sicilian organized crime figures involved with heroin trafficking and other racketeering operations.

Attorney General William French Smith, meeting yesterday for the first time with members of the new Italian-American Working Group on Organized Crime and Drug Trafficking, said the Italian operation is the "single most devastating assault on the Mafia in its entire history. . . . I think it's going to have a very debilitating effect when you combine what is happening in Italy and what is happening in the United States."

Rudolph W. Giuliani, the U.S. attorney in Manhattan who has worked closely with Italian law enforcement officials on the case, called the operation "a very significant breakthrough. We are able for the first time to get a picture of how the Sicilian Mafia is organized."

Giuliani said cooperation between the U.S. and Italian governments on the case was "unprecedented" and included not only dealings between high-level officials but also exchanges of agents and prosecutors.

He said the charges developed by Italian prosecutors involved murder, narcotics trafficking and associating with the Mafia, a crime in Italy.

Giuliani said most of those sought in the United States are members of the Joseph Bonanno crime family and include Salvatore Catalano, who heads one of its factions.

The Italian crackdown was sparked by a major heroin case, known as the "Pizza Connection," brought by Giuliani's office last April and by information provided by Tommaso Buscetta, a major organized crime figure who was extradited to Italy from Brazil last July.

Dennison Young Jr., the associate U.S. attorney in Manhattan, said 22 of the 28 sought for extradition were under indictment in the "Pizza Connection" case. Prosecutors have said the defendants used pizza parlors in New York and the Midwest as a cover for smuggling "billions and billions of dollars" in heroin into the United States over a five-year period.

Of the 22 three are fugitives, three are in prison and 16 are free on bail of as much as $3.5 million. The 16 were expected to turn themselves in for arraignment yesterday or in the near future in New York, New Jersey, Illinois, Michigan and Wisconsin.

The other six persons sought by the Italians have not been identified.

Young said a central figure among those sought by the Italians is Gaetano Badalamenti, 50, a close associate of Buscetta and a key figure in the "Pizza Connection" case. Badalamenti was arrested in Madrid last April and has been ordered extradited to the United States.

In an interview on CBS's "Morning News" program yesterday, Smith said the pizza case is "the largest heroin operation that has ever been broken in this country, and there is a close connection between that and what is happening now in Italy." Smith said the U.S-Italian action is more important than the famous "French Connection" heroin case "for the very reason we were able to arrest the very top leaders of these criminal syndicates and, being able to do that, we are able to interfere substantially with drug trafficking networks that bring heroin into the country."

The joint working group, which will finish two days of meetings here today, was formed last year by President Reagan and Prime Minister Bettino Craxi of Italy as part of a new drive on organized crime in both countries.

Under the new extradition agreement, charges against the suspects were forwarded to the Justice Department, and Italy will have 45 days to submit formal charges against those being sought for extradition.

Special correspondent Sari Gilbert reported from Rome:

The joint working group, according to Italian Interior Minister Oscar Luigi Scalfaro, "represents an unprecedented commitment for internationally broadening the attack on drug trafficking and organized crime."

Scalfaro, who headed the team of Italian officials that went to Washington for the first meeting of the group this week, said in an interview last week that, after years of watching the drug problem from the sidelines, Italy -- with an estimated addict population of between 100,000 and 200,000 -- "is facing a grave domestic problem."

Although in the past, rivalry between Italy's various police forces may have interfered with U.S.-Italian cooperation, Scalfaro promised that the new working-group arrangement will be based on "a total exchange of information and intelligence."

Despite continuing crackdowns by Italian police, between 50 and 65 percent of the heroin that enters the northeastern United States is brokered by the Sicilian Mafia or produced in clandestine Italian narcotics refineries, Italian and U.S. police sources said.

Most of it originates in Asia. But Sicilian Mafia members and their U.S. counterparts reportedly are deeply involved in the westward heroin trade and have been so ever since the destruction of the Marseilles-based "French connection" in the early 1970s.

The new agreement comes at a time when Italian-American relations are excellent. Cooperation in the 1982 release of kidnaped U.S. Brig. Gen. James Dozier from his terrorist captors was catalytic, U.S. sources said. A new extradition treaty was ratified recently and new tax and legal treaties have come into force.

According to Alberto Sabotino, head of Italy's antidrug force, in recent years Italian police have broken up 27 Mafia narcotics gangs in Sicily and put 800 people behind bars. At least eight clandstine laboratories in which morphine base was made into heroin were shut down between 1979 and 1981, and so far this year 830 pounds of heroin have been seized.

Sabotino said that Italian police were becoming convinced that the Sicilian Mafia is no longer deeply involved in the production of heroin.

Recently, Italian and U.S. agents cooperated on several major antidrug operations, including the arrest of Buscetta in Brazil late last year, the arrest of Badalamenti in Spain in April and, four weeks ago, an operation involving U.S. and Italian undercover agents that led to the arrest of several Mafiosi.