New Jersey Attorney General John Degnan said today-in what apparently is an unprecedented calm-that his prosecutors "are now prepared to prove in a court of law the existence of a national criminal conspireacy."

Degnan made the assertion in announcing a lengthy indictment against eight alleged members of the Vito Genovese organized crime family of New York. The men are charged with murder, extortion and conspriracy.

The existence of a "Mafia" or a "La Cosa Nostra" has long been established in American folklore, books and movies, as well as public testimony by former organized crime members. But law enforcement authorities said today they know of no instance where it has been proved in court.

In announcing the indictment, Degnan said: "Over the course of the last 25 years there have been many disclosures concerning organized crime . . . but there have always been lingering doubts in the minds of some public officials, our citizens and even a few law enforcement officials that there was such a national conspireacy."

The indictment alleges that the conspiracy ranged across New Jersy and New York to Nevada and Florida.

The charges are the first in a two-year combined federal-state investigation that focused on reputed New Jersey crime boss Anthony Russo, who was shot to death last month.

According to the charges, the conspirators killed or assaulted people to avenge insults and punish those who had violated the orders, laws, and protocol of "this thing of ours."

Ed Stier, a top Degnan aide, said the workings of the organization were described to show the supervision and control of the leaders. The evidence was gathered from court-authorized wiretaps and at least one informant.

Stier, director of New Jersey's division of criminal justice, said "this is the first time this kind of detailed information has been usable as evidence."

The indictment alleges that the reputed mobsters were part of a "secret nationwide criminal organization" which was known by its members as "this thing of ours" (La Cosa Nostra).

The indictment also charges specifically that defendant Ruggerio (Richie the Boot) Boiardo is a top offical of the Genovese family.

Recent federal indictments of top reputed organized crime figures have not referred to specific "families" or named the national organization.

Stier emphasized that the FBI and Justice Department organized crime strike force in New Jersey had aided the New Jersey state investigation. "We couldn't have done this without them," he said.