Red Brigades terrorists today murdered a Rome prison guard, dashing hopes that the arrest this weekend of a man police claim is one of Italy's most wanted urban guerillas and several of his alleged accomplices might have dealt a death blow to armed subversion here.

Police said Raffaele Cinotti, 28, was shot dead as he left for work at the prison. His death came only three days after antiterrorist police in Milan arrested Mario Moretti, a reputed long-time member of the Red Brigades thought by police to have been the mastermind in the 1978 kidnapping and murder of former prime minister Aldo Moro and to have been involved in 10 other murders.

Documents left at the scene of today's shooting called for the destruction of Italy's jails and the "liberation of proletarian prisoners," the same themes that dominated the month-long kidnapping of Rome judge Giovanni d'Urso earlier this year.

But in a telephone call to the Rome daily, La Repubblica, the Brigades warned Italian authorities, "Do not touch our comrades arrested in Milan." The four arrested -- Moretti, Genoese literature professor Enrico Fenzi and a man and a woman police describes as minor figures -- are currently being interrogated by magistrates from several Italian cities. They are expected to be tried almost immediately for illegal possession of arms.

The arrest of Moretti, a former radio technician who has eluded police for nine years -- immediately raised hopes here that the strength of the Red Brigades had been irrevocably compromised.

"This is the happiest day of my term as interior minister," Cabinet member Virginio Rognoni told reporters Sunday when confirming the arrests. A front-page editorial in Repubblica on the same day said "courage and firmness have triumphed."

Excitement generated by Moretti's arrest was due in part to the fact that "repentant" jailed terrorists such as Patrizio Peci have named him as a member of the Red Brigades "strategic command."

Furthermore, police in recent years have arrested more than 2,000 suspected or convicted terrorists, and in recent months the number of terrorist attacks has declined sharply. The Red Brigades' last murder was in January.

Police say that Italy's second-largest leftist terrorist group, Prima Linea, or Front Line, has been almost destroyed. Terrorist documents have also disclosed that the Red Brigades' organization is split between militarists and a more politically minded faction that is concerned with attempts to generate popular support.

Police sources estimate that the Red Brigades organizations in Turin and the central Italian region known as the Marche have been almost destroyed. They also believe that the organization's other branches -- in Genoa and the Veneto region -- are operating at about half strength while the units in Rome and Milan are somewhat stronger.

One of the first to try to minimize the import of Moretti's capture was Renato Curcio, a jailed Brigades leader, who told reporters that "the arrest of Moretti is irrelevant." Moretti and Fenzi have declared themselves political prisoners and are reported to have refused to answer questions. But Italian press reports claim Moretti told magistrates, "I am not what you think."

Because of his experience and his reputed role in the organization, Moretti's arrest "cannot but create enormous organizational problems for the Red Brigades," a Rome judge said. "It helps us to destroy their myth of invulnerability."

But the judge, who asked not to be named, pointed out that authorities cannot rest until all the top terrorists have been arrested. Among those police still report are at large are Moretti's girlfriend, Barbara Balzarani, and Frenzi's brother-in-law, Florentine criminologist Giovanni Senzani, who are believed to belong to the strategic command.