South African Gerrie Coetzee stopped former heavyweight champion Leon Spinks after just 2:03 in the first round of their World Boxing Association semifinal elimination bout today.

Three clubbing right hands from the 24-year-old each smashed Spinks to the canvas. The final blow sent Spinks tumbling against the ropes. Although Spinks got up, his eyes were glazed and referee Isidor Rodriguez of Venezuela stopped the bout.

Coetzee will meet American John Tate for the WBA title, which Muhammad Ali is expected to give up officially next month.

Only 1,500 spectators witnessed the upset in a makeshift stadium on a parking lot beneath the cliff where the castle of Monaco's ruler, Prince Rainier, stands.

Of these, 320 were South Africans. At the finish they tried to invade the ring to salute their hero, but scores of Monte Carlo riot police prevented them from reaching the canvas. The South Africans had jeered "Ali, Ali" at Spinks as he came into the ring for his first fight since he lost the title on a decision to Ali last September.

Spinks, a 25-year-old Olympic light heavyweight champion from St. Louis, was expected to win easily in spite of weighing only 200 pounds to Coetzee's 220 3/4.

Spinks swarmed into the attack at the start, landing a cluster of left jabs and right crosses.

Coetzee, although startled at the ferocity of the attack, backpedaled and avoided the worst of Spinks' rushes.

At that point Coetzee spoke to Spinks, but the former champion just laughed, although the laugh did not last long. The South African, unbeaten in 23 professional fights, landed a vicious right hook and Spinks, although up at four, had to take a mandatory eight count.

Coetzee then powered forward, jabbed accurately with his left and again floored Spinks with his crushing right cross.

Again, his opponent had to take a mandatory eight count.

By this time, Spinks seemed unable to move properly. Another blow by Coetzee sent Spinks reeling against the ropes. The referee stopped the fight as he has to do under WBA rules, which state that any boxer knocked down three times in one round is not allowed to continue.

According to the chairman of the WBA, Mike Mortimer, the fight between Coetzee and Tate will take place before the end of the year.

Coetzee said immediately after the fight that he did not expect such a quick finish.

"I was in great shape, but I really did not expect this," Coetzee said. "I am looking forward to John Tate. I thank Spinks for having proven to myself and to the world how good I am."

Spinks record dropped to 7-2-1.

Former European Heavyweight champion Alfredo Evangelista of Spain withdrew from his scheduled 10-round preliminary against Clifford Stephens of the United States, blaming a painful rib. Boxing sources speculated Evangelista's last-minute withdrawal probably was due to disagreements with his Spanish manager, Juan Berrocal. The two are known to be at odds over the handling of the fighter. CAPTION: Picture, South African heavyweight Gerrie Coetzee heads for corner as Leon Spinks sprawls against ropes. It was one of three times the ex-champ was floored in first round. UPI