ROME, SEPT. 26 -- A senior Sicilian judge and his handicapped son were ambushed and slain by Mafia gunmen late yesterday as they returned home to Palermo from a weekend in the country, Italian police said today.

The murders, which shocked politicians and law enforcement officials across Italy, appeared to signal a new underworld offensive against judicial authorities at a time when Mafia activities have been the target of an unprecedented crackdown.

Last year, in a widely publicized mass trial, 338 prominent Italian gangland figures were convicted and sent to jail.

Antonio Saetta, 66, and his 35-year-old son Stefano were killed in a burst of automatic-weapon fire as they traveled by car on a two-lane country road near the Sicilian town of Caltanisetta. Authorities said the gunmen pulled alongside the Saetta family sedan in a stolen BMW and opened fire.

Saetta and his son died of multiple wounds, police said, adding that numerous 9-mm shell casings were recovered from the scene.

Saetta was the eighth Sicilian judge to be slain by suspected Mafia gangsters in the last 20 years. He gained wide prominence in 1985 when he presided over the trial of two Mafia bosses, Michele (The Pope) Greco and Salvatore Greco, who were convicted of masterminding the murder of a judge who was investigating the Greco family. Saetta sentenced the brothers to life in prison.

Authorities were not certain whether yesterday's attack represented a personal vendetta against Saetta or a wider attempt to influence other reformers in Sicily.

Earlier this month Alberto Giacombelli, a retired criminal judge, was slain in a similar ambush in eastern Sicily, but authorities have yet to establish a Mafia link to that killing.

Italian Interior Minister Antonio Gava convened an emergency meeting in Palermo, the Sicilian capital, to discuss the Saetta slaying and evaluate possible countermeasures. Italian President Francesco Cossiga, meanwhile, vowed "the solid and immovable determination, of the people and institutions, to affirm the sovereignty of the law and the republican order everywhere and against any threat."

Yesterday's killings occurred just nine months after the historic mass trial of the 338 gangsters last December. They also coincided with a period of unprecedented disarray and squabbling among the various political and judicial bodies investigating mob actions.

In August, long-simmering disagreement over the handling of Mafia investigations between Giovanni Falcone, the Italian judiciary's top Mafia expert, and new Palermo prosecutor Antonio Meli, publicly erupted. Allies of Falcone charged that Sicilian law enforcement officials were incompetent and meddlesome.

Falcone threatened to resign but agreed to stay on after a last-minute intercession by the the judiciary's self-governing body, which promised him a certain amount of autonomy in his inquiries. The Italian government also assigned the nation's top antiterrorism investigator, Domenico Sica, to coordinate various fronts in the war on Italian organized crime.

Still, critics have charged that the bureaucratic bickering has allowed criminal organizations the time they needed to regroup.