A former New York prosecutor who became a mob lawyer testified yesterday that the Mafia keeps a nationwide stable of "trusted" attorneys who regularly produce perjured testimony, arrange payoffs and even turn against their own clients if they show signs of cooperating with the government.

Heavily guarded by federal marshals, Martin Light leveled the charges in an appearance before the President's Commission on Organized Crime that officials said was the first direct account from "a lawyer gone bad" with the mob.

In fact, Light testified that until yesterday morning, his wife had been receiving $400 a month from Colombo crime family boss Carmine Persico to tide her over while Light serves a 15-year sentence for conviction of possession of heroin.

"I was trusted," Light said of a career that he said spanned 15 years of representing members of all five organized-crime families in New York. "I was expected to 'to do the right thing,' " he said.

"What does that mean?" Stephen D. Ryan, the commission's deputy counsel, asked.

" 'To do the right thing' is to protect the family -- to go back to the family and say, 'Watch out for this guy' or 'Watch out for that guy,' " Light replied. "You 'do the right thing.' It's a way of life."

With that, and a grant of immunity from prosecution, Light, tanned and nattily attired, launched into a gripping insider's biography that began with his Jewish upbringing in an Italian neighborhood in Brooklyn, joining a Democratic club to get a job with the Brooklyn district attorney's office after graduating from Brooklyn College Law School in 1962, and then supplementing his salary with a civil practice that came to include two Mafia clients involved in a car accident.

Spotted in a Luchese crime family hangout called the 19th Hole as a result of that case, Light said he was called on the carpet by a new district attorney, Gene Gold, and told to drop his private practice. Light said he left the DA's office instead, in 1969, and soon began doing legal work for a Colombo family "crew" headed by an up-and-coming mobster named Gregory Scarpa.

"This fella, Sammy Weiss, was a member of the 'crew' " even though he was never a "made," or formally initiated, member of the Colombo family, Light explained, "and my mother gave him my business card. She said, 'Sammy, with all the goniffs [thieves] you know' . . . . That night, Sammy got arrested and he had my card in his pocket."

With that debut, Light said, "little by little, I got friendly. I went to all the weddings, all the wakes, the christenings and the barbecues."

Asked how he knew who was a "made" member of the Mafia when he never was one, Light said that "everyone knew who the racketeers were" in his neighborhood when he was growing up. He also said he learned a lot from conversations, including one with Scarpa about a Mafia "sitdown," or conference of elders that is called to settle disputes between family members.

Scarpa, according to Light's testimony, told how he presented his side of the dispute and then was excused, not knowing "if he was going to live or die." He left the club where the sitdown had taken place and "as he was going around the corner he heard shots. The other fella got killed, right on the sidewalk," Light recounted.

As a mob lawyer, Light said he represented Mafia members and their associates in gambling, extortion, shylocking and murder cases, to name a few. He said he collected 80 to 90 percent of his fees in cash because "it's the best way to get paid."

How much did he report to the Internal Revenue Service?

Light grinned at the question. "It can't be held against me?" he asked. Assured that it could not, he replied: "Maybe 30 percent. I had to show something. I had a big house . . . . "

Individual clients always took a back seat to the mob's interests, Light testified. He said he has told clients to plead guilty to protect higher-ups, coached others to destroy subpoenaed records, delayed trials while mobsters arranged for the disappearance of government witnesses, and concocted false testimony in seeking new trials.

In earlier interviews with commission lawyers about perjury on the part of witneses and clients, Light estimated that in "99 out of 100" cases, the mob lawyer not only knew of the perjury but was active in suborning it.

"It's always what's best for the family that counts," he testified. "I would sit down the with the captain a Mafia caporegime or the consiglieri counselor and we would figure out what's best for the family."

For instance, Light recalled, when members of Scarpa's crew were occasionally subpoenaed to appear before a grand jury, Light would caution them to give only their names and addresses, assert the Fifth Amendment and get excused right away. Light would wait outside.

"What if they didn't come out right away?" he was asked.

"Then they were talking," Light said. "I'd say, 'What are you, crazy?' I'd tell Greg. I'd say, 'This guy's crazy. He was in there with the grand jury half an hour.' "

Scarpa then would "talk to the guy" and if he thought there was a lot to worry about, Light said, "they'd get rid of him."

"Would they kill you for what you're doing today?" he was asked.

"Without a doubt," Light replied. He resigned from the bar following his 1984 heroin possession conviction. Officials refused to disclose where he is serving his sentence.

Light also testified that two mob lawyers from New York, Gino Gallina and Robert Weiswasser, had been murdered "because they didn't do the right thing." Gallina was killed reportedly because he was suspected of being an informer, and Weiswasser because he allegedly kept some money from a client. Commission spokesman Art Brill said both were killed in the mid-1970s.

In regard to payoffs, Light testified only that on a few occasions he had "corrupted" judges, assistant prosecutors and police officials and that he had been present at meetings "where payments went from hand to hand." He also said mobsters, including Carmine Persico, used his law offices occasionally as "safe" spots for mob discussions.

"In all the major cities where there is an organized-crime family, there's a nucleus of lawyers who represent organized crime," Light said. "If you're not trusted, organized crime is not going to use you."