Cleveland Denny, former Canadian lightweight boxing champion, died early yesterday in Montreal 17 days after being knocked unconscious in a June 20 bout at the city's Olympic Stadium.

Denny, 24, lapsed into a coma after an onslaught of punches to the head sent him to the canvas in the 10th round of his nontitle fight with Canadian lightweight champion Gaetan Hart of Buckingham, Quebec.

He was rushed to Maisonneuve-Rosemont Hospital where he underwent brain surgery to relieve the pressure of a blood clot between the brain and the skull. But the damage proved to be irreversible, and Denny died about 3:30 a.m. yesterday without regaining consciousness.

The Denny-Hart fight was the first preliminary bout to the Sugar Ray Leonard-Roberto Duran World Boxing Council welterweight title fight.

A native of Guyana, Denny initially came to Montreal for the 1976 Olympics, but chose not to fight because of a boycott by black African athletes.

Staying on in Canada after the Olympics, Denny turned professional and until June 20 had lost only one of his 18 fights. That loss was to Hart for the Canadian lightweight title in 1978. Denny defeated Hart in their first meeting.

Denny was the second fighter to suffer serious injury in consecutive bouts with Hart. On May 7, Ralph Racine, 23, of Niagara Falls, Ontario was knocked unconscious in a bout with Hart and subsequently underwent surgery for a blood clot on the brain. Racine was in a coma for several days, but eventually regained consciousness. He is currently undergoing rehabilitative therapy in a Hamilton, Ontario, nursing home.

The June 20 knockout that eventually proved fatal came in the final seconds of the fight. Judges' score cards showed Denny only a few points behind Hart going into the 10th round, and Denny had won the eighth and ninth rounds.

But a flurry of savage punches 20 seconds before the final bell knocked Denny out. Despite the emergency surgery, the swelling was so severe that the brain stem was moved 15 centimeters.

Wired to life support systems that kept his heart beating, Denny had been clinically dead since Wednesday when three separate electroencephalograms showed no brain activity.

Initially, family members had decided to withdraw the life support systems, but changed their minds after Denny's mother Dorothy of New York said she thought she detected a slight improvement in the young fighter.

Doctors, however, had warned that Denny's chances for survival were hopeless, and they said that even with the life support systems death would come within a matter of days.

With his wife Clarine and his mother keeping a vigil at his bedside, Denny's condition worsened steadily over the weekend. His heartbeat flickered between 30 and 45 and his blood pressure sank dramatically.

Eventually, said his wife, his heart finally gave out.

"The inevitable has happened," she said, weeping. "They said it would happen, but I still kept hoping."

The father of a 6-month-old son, Cleveland Jr., Denny had fought and trained in California for two years before returning to Canada this spring.

His most recent fight had been a draw in Los Angeles, and he had worked as a machinist between fights.His wife, however, said boxing was always on his mind, and he was convinced that eventually he would win a world title.

Hart, 26, was said by his manager, Andy Nadon, to have been so deeply shaken by Denny's death that he had canceled a fight scheduled for Aug. 7.

Earlier, Hart had said that he sympathized with Denny's family.

But he added, "I hope they understand the situation. I have to earn my living like everyone."

Denny was the fourth boxer to die within the last year from injuries received while in the ring. Since 1946 almost 350 fighters have died from injuries received while boxing. In most cases, the cause of death is a blood clot in the brain caused by a heavy blow to the head.