LOS ANGELES (AP) _ To the associate who turned him in, Kevin David Mitnick is a ''big, fat slob'' obsessed with fast food and breaking into computer systems.

To the authorities holding him without bail, the 25-year-old Los Angeles man is an expert hacker who calls himself Condor and is so dangerous he must never be allowed to use even a telephone without supervision.

''He is truly a threat to society, someone who uses computers to act out personal vendettas against people,'' said Jim Black, a computer crime specialist with the Police Department.

''The bottom line with Kevin is that if you have a job, pay a gas bill, use a phone or drive a car he can find you and ruin you electronically,'' Black said in a telephone interview. ''If he wasn't in jail, he could be listening to us right this minute.'

Mitnick was being held today in a maximum-security cell at the federal Metropolitan Detention Center.

A federal grand jury indicted him Dec. 20 on computer fraud charges, accusing him of using unauthorized MCI long-distance codes to tap into computers at Leeds University in England and the University of Southern California and causing $4 million damage to a Digital Equipment Corp. computer system.

Mitnick will plead innocent, said his attorney, Alan Rubin.

''We will also try to get bail set,'' Rubin said. ''I am shocked and astonished that my client is being held without bail and in solitary confinement. This thing has been blown completely out of proportion.''

Longtime associate Leonard DiCicco, 23, of Calabasas, said he reported Mitnick's activities to authorities because Mitnick had been threatening him.

''He was using DEC computers where I work and threatening that we would both go down if I said anything,'' said DiCicco, who faces no charges in the inquiry.

''He has no sense of right or wrong and no respect for anyone else's privacy,'' DiCicco said. ''I was beside him through all this hacking and it's scary to think of the things he is capable of.''

''He could pick apart almost any computer system in the United States,'' Black said. ''There are probably only a handful of computer specialists with his capabilities.''

In his electronic activities, the 5-11, 240-pound Mitnick referred to himself as Condor, Black said. DiCicco said Mitnick named himself for Robert Redford's character's code name in the spy movie ''Three Days of the Condor.''

''All he did was eat Fat Burgers, drink Slurpees and work on computers,'' DiCicco said. ''He was a big, fat slob of a guy who couldn't get through a day without breaking into a computer somewhere.''

Mitnick began his hacking career using telephones, authorities said. By age 13, he was under investigation by the police department, Pacific Bell telephone company and the Federal Communications Commission, the Los Angeles Daily News said Monday.

''Mitnick was a legend to the computer world by 1981,'' said John Christ, his high school computer teacher.

In 1982, DiCicco said, he and Mitnick used USC's 24-hour student computer lab to get into many computers, including classified systems.

''Our favorite was the National Security Agency computer because it was supposed to be so confidential,'' DiCicco said. ''It was like a big playground once you got into it.''