Greg Page became the World Boxing Association heavyweight champion tonight when he knocked out Gerrie Coetzee with a straight left to the jaw in the eighth round at this casino resort in a segregated black reserve.

Page, 26, from Louisville, came from behind against Coetzee after lackluster losses to Tim Witherspoon and David Bey this year. He was only sixth in the WBA rankings and this week was removed altogether from the World Boxing Council rankings (he had been No. 5) for accepting the fight in view of South Africa's racial policies.

"You told me I was through. You told me I was washed up!" a jubilant Page shouted as he leapt from the ring and began dancing toward the dressing room.

Hundreds of local blacks joined in his celebration after defeating the white South African, Coetzee, in this racially divided society. They followed Page, waving their fists in the "black power" salute of African nationalists.

It was Coetzee's first defense of the title he won by knocking out Michael Dokes in Richfield, Ohio, 14 months ago. He has had a checkered career, including being knocked out by Mike Weaver in Sun City in 1980, and ringside opinion tonight was that he might now retire.

At a press conference after the fight, Page credited his trainer, Janks Morton, who used to help guide Washington, D.C., area neighbor Sugar Ray Leonard, for making him champion after a spell in the doldrums.

"They didn't beat me physically, I beat myself mentally," he said of his defeats by Witherspoon and Bey. "I was counted out, my career was at a standstill. But thanks to Janks, he got me thinking straight and I started fighting to my full potential again."

There was some question about the length of the final round. Coetzee hit the canvas after what appeared to be 3:45 of the eighth round. Well before Page dropped Coetzee, sportswriters at ringside were shouting at timekeeper Phil Swart that the three-minute round had elapsed. Page and Morton agreed it had been a "long round."

The scheduled 15 rounds began with both men throwing barrages of punches. Page weighed in at 233, more than 19 pounds heavier than Coetzee, and his punches were stronger, although he looked a little flabby around the midriff.

Coetzee's seconds were slow clearing the ring after the bell to start the second round. As the South African hesitated watching them, Page rushed across the ring and caught him with a stunning right hook to the head.

For a moment, the champion was in trouble on the ropes, but he recovered and the fighters slugged it out with straight lefts and right hooks for the next two rounds.

The real drama began near the end of the sixth round when a right from Page had Coetzee sagging. Three more blows after the bell, for which Page received a warning, and Coetzee sank briefly to his knees.

Page increased the pressure in the seventh and a flurry of combination punches put Coetzee down for a mandatory count of eight. But he recovered and twice drove a groggy Page into a corner, but by then his punches had lost sting and it looked as though both fighters might go down together.

In a supporting bout, Piet Crous of Johannesburg scored a major upset by taking the WBA cruiserweight (190-pound) title from Osvaldo Ocasio of Puerto Rico. Crous (23-0-1) was cut badly over his left eye in the middle rounds, but there were no knockdowns in the 15 rounds and he earned a unanimous decision.