Roberto Duran agreed yesterday to fight Sugar Ray Leonard in a bout of such appeal to boxing fans that it will be shown only on closed-circuit television in the United States, a medium previously used for heavyweight attractions.

Carlos Eleta, Duran's manager, said be telephone from Panama City that Leonard will defend his World Boxing Council welterweight championship on Friday night, June 20, "Maybe in Las Vegas."

Leonard is expected to receive at least $3.5 million and Duran $2 million.

Duran, former WBC lightweight titleholder, is ranked No. 1 welterweight contender for Leonard's WBC championship and for the World Boxing Association title held by Jose (Pipino) Cuevas. Duran has won 68 of 69 bouts, 54 by knockout.

Duran's only loss was to Esteban DeJesus, on a 10-round decision in November 1972. He atoned for that by knocking out DeJesus in the 11th round in defense of his lightweight title in 1974.

Leonard, 23, who won an Olympic gold medal as a light welterweight in the 1976 Games in Montreal, is unbeaten in 27 bouts as a professional. He scored his 18th knockout (fourth round) against David Green of England March 31 at Capital Centre.

Duran, who will be 29 on June 16, knocked out Joszef Nsubuga of Uganda in the fourth round Jan. 13.

Duran's manager said from Panama City: "Everything is set. Neither boxer will fight before June 20. Roberto has been in New York City since Sunday to begin training."

Dave Jacobs, Leonard's trainer, said here: "Ray has been running in the mornings to stay in halfway condition, because he thought he was going to fight Cuevas. We wouldn't want him to peak too soon. He'll be ready for Duran, you can be sure."

Eleta, a businessman in Duranhs native Panama, reported that he was able to get together with Leonard's attorney, Mike Trainer, and with promoters Don King and Bob Arum there. Janks Morton, trainer and adviser to Leonard, also was present.

"King is a very good friend of mine," Eleta said. "He will get a piece of the fight."

King has promoted several of Duran's big bouts. Rival promoter Arum had staged some of Leonard's, but attorney Trainer has kept his client free of ties to promoters for more than one but at a time.

When an impasse was reached in off-and-on negotiations for a bout with Duran, Trainer arranged a bout for May 16 between WBC champion Leonard and WBA champion Ceuvas.

But Cuevas suffered a cut eye in his last title defense, on April 15 while knocking out Harold Volbrecht of South Africa in the fifth round.

On March 25, Jose Sulaiman of Mexico, the WBC president, stipulated that Leonard would have to fight Duran before anyone else or be stripped of hiw WBC title.

Rodrigo Sanchez of Panama, president of the WBA, this week and reported as being under pressure from authorities in Panama to favor Leonard defending his WBC title against Duran, the WBC's No. 1 contender.

Trainer previously offered Duran $1 million to fight Leonard, the same amount Leonard received when he took the WBC title from Wilfred Benitez Nov. 30. Benitez received $1.2 million.

The combined purses for Leonard and Duran ($5.5 million) account for the bout going to closed-circuit, or theater, television. Conventional television networks probably would not risk that much on a sports event that could end in one round, and thus prevent them from showing enough commercials to recover their investment.

Duran's manager said the bout will be shown in other parts of the world on conventional television, another source of revenue to help cover the boxers' purses and promotional costs.

The exposure Leonard received on television during the Olympics, his personality, his stylish performances, hand speed and knockout punch have made him the biggest box-office draw in the sports since Muhammad Ali retired.

Duran has been an international favorite for 10 years because of his numbing punches and his disdain for finesse, although he is a wily fighter. He won the lightweight championship in 1972 from a superb boxer, Ken Buchanan of Scotland, knocking him out in the 14th round.

He abdicated his lightweight title in February 1979 because he had trouble making the 135-pound limit and because, as he put it, there were no capable opponents left in that division.

Ray Arcel, regarded as one of the wiesest figures in boxing, is Duran's trainer. He is not given to superlatives but he said in New York, "This is one hell of a . . . hell of a fight. Both fighters are well-skilled. Roberto is the best fighter in the world today. He would fight a cage full of lions.

"Leonard is an excellent fighter, but he hasn't reached his peak yet. The worst thing that can happen to Leonard is that he can lose, but he can recover from that and go on to big things.

"Roberto never got the press that Leonard does because Roberto is not American. The writers do not speak his language. The insiders know how good Roberto is. That is why he and Ali are being honored as the 'fighters of the decade' next month at the New York Boxing Writers' dinner.

"Roberto grew up in the streets of Panama. He would go into bouts with opponents and he didn't even know who they were, and didn't care. He had to go all over the world to get guys to fight him. Everybody wanted a fortune just to get in the ring with him."

Why did Duran let himself balloon up to 170 pounds before he fought, and looked unimpressive in decisioning, Zeferino Gonzales in September?

"He couldn't work himself into a fighting mood against what he knew was a tomato can (journeyman). Roberto is past that stage now. Both of these guys will be well paid. Where else could Leonard pick up $2.5 million to $3 million?

"Roberto does get a little despondent. He won 10 of 10 rounds from Carlos Palomino (former WBC welterweight champion).

"Then Benitez wouldn't get in the ring with him. Cuevas wouldn't fight Roberto for $1 million. The first thing a fighter loses is his ambition. He says, 'Ah, the hell with it,' especially if he has a little money.

"Leonard wanted to fight Benitez again, I heard. I listened and laughed.

I know Angelo Dundee (Leonard's second) is a smart fellow. He had to match Leonard where he could make a lot of money, get rich quick. That's what all fighters want. For this kind of money, Roberto will be ready."