Two anti-Castro Cuban exiles thought by federal investigators to be centrally involved in the alleged conspiracy to murder former Chilean ambassador Orlando Letelier were arrested yesterday in Miami and held on charges unrelated to the 1976 bombing of Letelier's car.

One of the men, Guillermo Novo, has been a fugitive for nearly a year after federal investigators probing the Washington assassination of Letelier began to investigate Novo's foreign travel and found he had traveled to Chile and Venezuela without informing his probation officer.

The other, Alvin Ross, was subpoenaed before the grand jury probing Letelier's death here last spring and was granted immunity from prosecution if he would cooperate with law enforcement authorities. He still refused to talk, however, and was never presented to the panel as a witness.

According to an announcement by the Miami bureau of the FBI, the two men were arrested separately yesterday and there was no indication that either was aware of the other's presence in the city.

The arrests are the latest in a series of developments in the Letelier investigation bolstering investigators' hopes that charges may ultimately be filed in connection with the murder case, according to one FBI official.

Investigators are known to believe that the bombing was carried out by anti-Castro Cuban exiles acting under the orders of representatives of Chile's secret police unit, know as DINA. Last weekend, Chile expelled an American-born DINA agent, Michael Vernon Townley, who reportedly had traveled to the United States to meet with certain bombing suspects about a month before Letelier's murder.

Letelier and an aide, Ronni Moffitt, were killed when their car exploded on Sheridan Circle NW on Sept. 21, 1976.

Novo, a top official of the Cuban Nationalist Movement in the United States, did not show up at a New Jersey federal court hearing last June 6 concerning possible violation of his probation on a 1974 explosives conviction.

Assistant U.S. Attorney Eugene M. Propper, who has been coordinating the extensive investigation into the Letelier murder, said at the time that the probation should be revoked because of his allegedly illegal travel to South America.

Novo and his brother were charged in 1964 with firing a bazooka toward the United Nations when Che Guevara was speaking there, but charges were dropped because they were not read their rights by the police officers who arrested them.

In 1974, Guillermo Novo was convicted in connection with a conspiracy to destroy the Cuban consulate, Cuban trade commission and a steamship belonging to Cuba - all then based in Montreal, Canada. It was for that conviction that he received the probation term he is now accused of violating.

The FBI said Novo, 38, of New Jersey, was arrested at a Miami motel. No further details of the arrest were available last night, although there were reports that guns and cocaine also were found at the time of the arrest.

Ross, 45, of Miami, was arrested in his car at the exit ramp of an expressway. He was charged on a New Jersey warrant with conspiring to make explosives and other destructive devices and storing high explosives in violation of Federal laws.

Another person was arrested with Ross, the FBI said, but his correct identity has not been determined. Novo is being held without bond, and a $50,000 bond has been recommended for Ross.