Twenty-four hours after the largest narcotics raid in Cambridge history, "Marley Wilson" was out on bail and watching sidewalk traffic in Holyoke Center.

"They didn't even dent heroin sales," Wilson said yesterday. "They thought they did, but they didn't."

Twenty-eight arrests--the culmination of four months of undercover investigation by Cambridge and State Police detectives--centered on the Harvard Square, Central Square, and Western Avenue sections of Cambridge.

"It wasn't just for heroin," Cambridge Captain Thomas Burke said yesterday. "It was about 60 per cent heroin--the rest was other drugs." He said that the total amount of drugs seized had not yet been determined.

Wilson--who declined to go into particulars of his case--said that the arrests had been made on the basis of purchases by undercover police agents posing as addicts.

"Most of the stuff the police bought was phoney--they were just phoney bags," he said. "I imagine they'll change the charges to larceny or fraud."

Cambridge Police Chief James J. Reagan said yesterday that it would be "premature" to speculate on the nature of the materials bought. "They have to go to the laboratory to be analyzed first," he said.

A bystander, sitting on a ledge five feet away from Wilson, said that much of the heroin around came from police officers who sell what they get in raids.

Burke called the charge "false."

"Hopefully, these raids have slowed the heroin traffic down," he added. "By how much, it's hard to say."