When the U.S. soccer team tried to warm up before its Pan American Games debut against Guatemala this morning, it was ordered from the field for an identification check.

When the U.S. substitutes practiced following the scoreless first half, the public-address announcer several times demanded, "Attention please, will the United States players get out the field? Thank you."

The principal indignity was yet to come. The lightly regarded Guatemalans all but ran the United States out of Estadio Nacional Brigido Iriarte with a 3-0 victory that delighted the crowd of 7,000 and shocked a U.S. team that believed it had been progressing steadily toward a respectable showing in the 1984 Olympics.

"We scrimmaged them yesterday and we thought we were a better team," said goalkeeper Tim Harris of UCLA, so frustrated by his lack of support that he left the field without participating in the traditional postgame handshake. "Any time you lose, it's disappointing, but we sure had higher expectations in this one."

"They weren't good. We were bad," said Coach Manfred Schellscheidt. "In the second half, we were sleepwalking. We looked sluggish and I don't know why. Maybe it was playing at 10 o'clock; maybe it was the strange heat that makes it hard to breathe; maybe it was the schedule change. But that was probably the poorest game since we brought the team together."

The United States had been scheduled to play top-ranked Brazil today, but the draw was revised Sunday and it was placed in a group with Guatemala, Cuba and Chile.

Since only the winner of each group qualifies for the semifinal and final, any chance of a U.S. medal seemingly disappeared today.

"For weeks we knew we would play Brazil, and we felt the match would show us where we stand for 1984," Schellscheidt said. "Then they changed the whole thing and maybe we thought this was easy. But no international match is easy.

"After they scored, we were very nervous and made a lot of mistakes. Maybe we need to grow up to the pressure. That's why we're here. This is a level up from what we play at home."

Guatemala opened the scoring after 55 minutes when a shot by Walter Claveri deflected into the net off defender Kazbek Tambi.

Michael Fox, who scored twice in the qualifying victory over Canada, had a chance to tie but missed the far post from the right of the box. Jose Bobatilla and Julio Gomez then wrapped it up while the U.S. defense broke down and Guatemalan flags waved from both sides of the stadium.

"I don't think they're any better than we are," Fox said. "They scored a goofy goal and I blew a chance to tie it. We were a bit slow and I don't know why, but we're getting experience and I don't see any big gap between us and anyone else."

Fox, a midfielder, played at California State-Fullerton. One of 18 players on the Cosmos' main roster, he remains an amateur to be able to participate in the Olympics.

"The Olympics are a once-in-a-lifetime thing and that's why I'm staying amateur," Fox said. "After they're over, it would be a good advancement for me to join Team America and aim for the 1986 World Cup.

"I like the idea of Americans playing together, but I also like the idea of playing with guys like Bogie (Vladislav Bogicevic) and (Roberto) Cabanas on the Cosmos. If I would start for the Cosmos, I might get more experience staying there."

The U.S. team did not participate in the opening ceremony Sunday because of today's early start.

"We didn't want to be walking around all day yesterday," Fox said. Instead, they apparently saved the walking for today, while the Guatemalans kept them on the run.