They came early and remained in the muggy De Matha gymnasium until almost dark, watching the tall young men twirl and whirl, shoot and stuff and demonstrate conclusively why they are the best high school basket-ball players in the country.

The occasion was the opening practice for the U.S. All-Star team that will play its Washington counterpart in the Capital Classic Thursday night at Capital Centre. And many of the great and near-great among America's college coaches were there to watch.

Jerry Tarkanian, fresh from his Nevada-Las Vegas team's third-place finish in the NCAA tournament Monday night, sat in the bleachers and chewed on his fingernails as if a national title was at stake. With players of this stature involved, it is.

Norm Sloan, the North Carolina State coach, chomped on his chewing gum and watched every move of big Wayne McKoy, the 6-foot-10 center from Long Island that Sloan covets dearly.

"There is a player here I won't name who is vitally important to our program," said Sloan, dapper in a gray leisure suit, open-neck shirt and tasseled, suede loafers.

"If you are not here, they might interpret that to mean you are not interested, so our presence certainly can't hurt. Yes, we just want them to know we're here."

A few rows back sat Michigan coach Johnny Orr, although he admitted he was not especially thrilled to be in downtown Hyattsville this lovely spring day.

"I'm not real big on these all-star games," he said. "Actually, this is the first one I've seen in four years. I think they're exploiting players in a way. These things make it tougher to deal with the kids, tougher to get them back down to reality later on."

Still, Orr and two of his assistants were present to send the appropriate "we-want-you" message to big Earvin Johnson of Lansing, Mich., even if the rules prohibited Orr and all of his colleagues from talking with the players. If they didn't know it already, a large sign - "No Coaches Allowed" - was taped on the locker-room door.

Lefty Driesell had another approach. Each of the 12 U.S. players have been accompanied to the game by his high school coach, so Driesell and two assistants went one on one with anyone who cared to listen out in the corridor near the De Matha trophy case.

Georgetown's John Thompson stayed for an hour then left to watch the Washington-area players at Blair High. "I can't compete with all these guys with the bigh budgets and all the airplanes," Thompson said with a sly smile.

"But I'm here because you just might get lucky. You might hit one time, so I come and watch. I'm like anybody else. I love to see a good player."

And most of the good ones - Johnson, McKoy, Eugene Banks of West Philadelphia, Albert King of Brooklyn, Darnell Valentine of Wichita - are here.

They practiced for two hours yesterday, a session highlighted by an in-you-face, sky-high, double-pump, above-the-rim jamming contest in what was supposed to be a routine drill.

"There are eight franchises out there," said Howard Garfinkel, director of an extensive high school scouting system and the man who will introduce each player to the crowd Thursday night.

"Most other years, you see one franchise. If you lie a little, maybe two. But I'm telling you there are eight kids out there who can turn eight different programs around right now. You know they want me to limit my introduction to 30 seconds on each kid. No way I can do that. I got to go at least 45 and even that cuts out the good shtick."

Most of the players and their coaches seem to enjoy the all-expenses-paid, three-day trip, although one coach said, "I'm very much opposed to these things. There are a lot of pimps running around out there." Nevertheless, classic officials go all out to show their guests a good time. There was a tour of the White House and Capitol Hill, a luncheon at the Capitol Hill Club and a dinner at the Sheraton Lanham last night, with John Wooden the featured speaker.

"No I don't feel exploited," said Eugene Banks, considered among the top three players in the country. "Maybe when I'm a little older, I might look back at it that way.But it's a chance to play with the best guys and to get to know them as people."

Banks already has signed an ACC letter of intent ot attend Duke and, for the most part, the recruiting pressure is off. Still, there was what his coach Joey Golden bert, described as a "disgusting incident" at the Knight of Columbus tournament here two weeks ago that Goldenberg does not want to see repeated.

Digger Phelps, the Notre Dame coach, came out of the stands with several of his former All-America players from Washington and just happened to bump into Banks after one of the tournament games. Phelps stopped for a chat while a Duke assistant did a slow burn nearby. The aide knew that Phelps was still pitching and wooing Banks for the Golden Dome, even though he had already signed to head south.

"It made me sick," Goldenberg said, "I didn't even want to talk to Digger after it happened. I just hope nobody bothers Gene here. He's going to Duke and it should be all over."

Perhaps that is why Duke assistants were also at De Matha yesterday, and everywhere Goldenberg went, a Duke man was sure to follow.

"Even after Gene told the Duke people he was going to go there, they didn't miss a game," Goldenberg said. "It's all part of the business."

And that is why attendance in a tiny sweatbox of a Hyattsville gymnasium was mandatory yesterday. As Sloan admitted. "Maybe I'd rather be someplace else. But if you're a coach, this is where you have to be."