The new Olympic track in the Coliseum opened for competition today, and one of the first athletes to test it was Alberto Juantorena of Cuba, who won the 400 and 800 meters in the 1976 Olympics at Montreal.

Juantorena dawdled in fifth place most of the way during an 800-meter heat in the International Summer Games. Then he used his long legs to overtake the opposition around the last turn and he cruised home second in 1:48.94, easily qualifying for Sunday's final.

"The track is marvelous, I'm very impressed," Juantorena said in an interview conducted entirely in English, after he had first hugged old friends and obliged autograph seekers. "It is very fast and it is soft, good for my old legs."

Juantorena, at 32, ran his fastest 800 since 1977 last week in Havana, when he was timed in 1:44. He also tried the 1,500 for the first time and clocked 3:45.

"I am getting old and losing speed, but I am increasing my resistance," Juantorena said. "The 1,500 was very difficult--too long, 700 meters longer than I have run before. I will concentrate on 800 meters in the Pan American Games and the World Championships, but starting in October I will work harder on the 1,500."

Juantorena said he was in "very good condition," and he looked it. At 6 feet 3 and 182 pounds, he is four pounds lighter than when he began his memorable nine-race grind over seven days in Montreal. He also is running more and said he would cover 2,000 kilometers (1,250 miles) in training this year, compared to 1,500 kilometers (937 miles) in 1976.

"I'm running more distance than ever before and I'm enjoying it more," Juantorena said. "I'm fighting it in myself and I'm winning."

This was the first track competition in the Coliseum since 1974 and stadium records fell with frequency.

Maria Petkova of Bulgaria started by winning the women's discus at 232 feet 1 inch, more than 48 feet beyond the 1964 mark of Soviet Tamara Press. Mayra Vila of Cuba took the women's javelin with 208-3, adding 22 feet to the record.

Silke Gladisch of East Germany won her 100-meter semifinal in 11.23, another stadium mark. It is one that does not figure to last long. That event is the highlight of Saturday's opening program in the U.S.-East Germany dual meet, with world record holder Marlies Gohr and Marita Koch opposing Evelyn Ashford and Diane Williams.

Other record setters were Shigenobu Murofushi of Japan, 241-4 in winning the hammer throw, and Dagmar Ruebsam of East Germany, 50.97 in a women's 400-meter semifinal.

Another important confrontation Saturday features Tom Petranoff, who holds the world javelin mark of 327-2, against East Germany's Detlef Michel, who recently matched history's second-best throw of 317-4.