Former gangster Stephen “The Rifleman’’ Flemmi testified Thursday that he inadvertently stumbled upon a murder decades ago at the Sharon home of New England Mafia boss Francis “Cadillac Frank’’ Salemme.

Flemmi said he was parking his car outside the Marie Avenue home on the morning of May 10, 1993, hoping to find Salemme inside, when he spotted South Boston nightclub owner Steven DiSarro walking toward a back door with Salemme’s son, Frank. He said he followed the pair and found the door leading into the kitchen slightly ajar.

“I opened the door to walk in,’’ Flemmi said. “That’s when I saw Salemme Jr. had Steve DiSarro by the throat. He was strangling him. Paul [Weadick] was holding his legs.’’

Flemmi said the elder Salemme, who was standing nearby, “immediately stepped to the threshhold’’ when he saw him.

Flemmi said he told Salemme, “Frank, I’m leaving. I’ll see you later,’’ then left immediately.

It was Flemmi’s second day on the stand at the murder trial of Salemme, 84, and Weadick, a 62-year-old plumber from Burlington. They are charged with killing DiSarro, a 43-year-old father of five from Westwood, to prevent him from cooperating in a federal investigation targeting the Salemmes. DiSarro and his stepbrother had purchased the Channel nightclub and several witnesses testified that the Salemmes had a hidden interest in the business. Salemme’s son, Frank, died in 1995 of lymphoma.

Flemmi, who will turn 84 on Saturday, said he didn’t know DiSarro would be killed that day, but knew his life was in danger.

Shortly before DiSarro’s slaying, Flemmi said Salemme told him he “had information’’ that DiSarro had been meeting with a federal agent and he was concerned that the nightclub owner might implicate the Salemmes in criminal activity.

“Frank wanted to kill him,’’ Flemmi said.

At the time, Flemmi said his son, Bill St. Croix, was friendly with Salemme Jr. and had been doing construction work at the Channel. He said he warned his son to “stop hanging around’’ with people at the club “because something may happen to Steve DiSarro.’’

Flemmi is the first and only government witness who claims to have witnessed DiSarro’s slaying. However, two Rhode Island mobsters, brothers Robert and Joseph DeLuca, previously testified that Salemme enlisted them to bury DiSarro’s body behind an old mill in Providence.

Years later, Salemme began cooperating with the government, helped send corrupt former FBI agent John J. Connolly Jr. to prison, and was placed in the federal witness protection program. He was living in Atlanta with a new identity when DiSarro’s remains were found two years ago, leading to the current murder charges.

Flemmi teamed up with Salemme on crimes beginning in the 1960s, then partnered with notorious gangster James “Whitey’’ Bulger in the 1970s while Salemme was in prison. For decades, Bulger and Flemmi got away with murders while secretly working as FBI informants.

In 1992, according to Flemmi, the Salemmes and DiSarro were facing neighborhood opposition to plans to feature nude dancing at the Channel, which was being transformed into Soiree. He said Salemme asked him to enlist Bulger to “use his connections’’ in South Boston to help the club get approval from licensing officials.

Flemmi testified that Bulger resolved the licensing problem after Salemme paid him $5,000.

On Thursday, jurors heard gruesome testimony as Flemmi described how he, Bulger, and other gangsters shot and strangled people -- including two young women.

Flemmi is serving a life sentence for 10 murders in the 1970s and 1980s, but admitted he participated in about 50 slayings dating to Boston’s gang wars of the 1960s.

During cross-examination by Salemme’s attorney, Steven Boozang, Flemmi acknowledged he pulled the teeth of his victims to make them difficult to identify in an era when DNA testing had yet to evolve. He described shaving teeth into dust, then flushing it.

When Assistant US Attorney Fred Wyshak asked Flemmi why he left so quickly after witnessing DiSarro’s murder, Flemmi said he thought Salemme “could have been under surveillance.’’

Shelley Murphy can be reached at shelley.murphy@globe.com.