Alphonzo Allen's strong, slender arm ripped the basketball down off the glass backboard, and as soon as his high-topped leather sneakers hit the hardwood floor he streaked off downcourt.

In just a few strides, the tall, graceful youth in the red uniform had passed the midcourt stripe in the Banneker Junior High School gym, with only one white-uniformed opponent to beat.

Gliding to the basket, Allen faked left with his shoulder. His opponent lunged to block him, but Allen pivoted on his right foot at top speed and caromed the ball softly off the backboard and into the red, white and blue netting of the basket. His team went ahead, 14-11.

"He's gonna be something, that boy. He's only 13," said Ernie Moore, the 45-year-old director, scorekeeper and chief cheerleader of the city recreation department's holiday basketball tournament at Banneker, one of several such tourneys under way yesterday around Washington.

The tournaments give hundreds of city youngsters "something to look forward to," said Moore, especially some youths who haven't had the best Christmas. It is also a chance to play in an organized league with uniforms, scorekeepers, coaches and crowds, an experience that builds teamwork and discipline, he said.

But besides that, he added, "It's a real thrill."

Here, on the blond hardwood floors beneath the electronic scoreboard, before scores of cheering onlookers, baskets are scored and games won or lost. Not incidentally, dreams are dreamed.

Youngsters here talk about their idols with the magical names, Dr. J, Magic Johnson, the Bird. For the Banneker tournament's young ballplayers, there is precedent: Elgin Baylor, Dave Bing, Austin Carr and Adrian Dantley, to name a few National Basketball Association legends, all once played on Washington playgrounds. Georgetown University coach John Thompson, who played with the Boston Celtics, was a product of Washington-area schools. On occasion, said Moore, the big stars revisit the District, coaching kids and, by their example, inspiring them to hope that they too, can someday make it.

Moore, a burly Alabama native who has coached youth teams here for 20 years, said he has often heard the fantasies from the teen-aged would-be stars: "They say 'Mr. Moore, one day, you are gonna see me on TV. I'm gonna be a star.' And I say 'Well, maybe, but it's gonna take a lot of hard work.' "

Moore said he knows the dreams are long shots. "I try to tell them that education is number one. Homework comes first," he said.

Once the whistle blows to start the game, though, basketball comes first for youngsters such as Alphonzo Allen and the other red-suited members of the Stead All-Stars from the Stead Recreation Center at 1625 P St. NW.

Yesterday, the All-Stars took on the Banneker Bullets in the opening game of the Jerome Stewart Memorial Basketball Tournament, named for a Golden Gloves bantamweight boxer who attended city schools and later boxed for the U.S. Navy team. Stewart was killed in a plane crash with other American athletes near Warsaw, Poland, on March 14, 1980.

Alphonzo Allen is only a baby-faced 13, but he already stands nearly 6 feet tall, towering a head taller than any of the youngsters in his age group. He has broad shoulders and slender legs that are slightly bowed but well muscled.

He also has the moves: the turnaround jump shot from 10 feet, with the sudden pirouette in midair that leaves slower opponents earthbound; the quick outlet pass that he sometimes can flick off his fingers to a teammate without even looking; on defense, the menace of constantly raised arms and the well-timed leap that blocks opponents' shots with a loud smack of hand on leather.

By halftime in yesterday's game, Allen, playing center for Stead, was dominant. He had scored eight of the team's 14 points at the half, pulled down about a dozen rebounds, and intimidated his opponents into missed shots and mistakes. But the Banneker team stayed within striking distance.

During the intermission, Stead got a pep talk from coach Eddie Parker, 18, a 1981 graduate of Ellington School of the Arts. Parker said he is currently out of work but plans to get a job in food service next month. He volunteers to coach, he said, "'cause I got nothing better to do . . . . Nah, I'm just kidding, I really like it."

Parker told the five sweating Stead All-Stars to tighten up on defense and work the ball inside to Allen on offense. Allen, who frequently passes to his teammates, complained that they aren't shooting enough, "Y'all are scared to shoot," he muttered between breaths. His little brother Anthony, a year younger and almost a foot shorter, told him to shut up and keep passing.

Then, early in the second half, the white-shirted Bullets made a comeback. Allen slipped and fell on defense, and later he missed an easy layup. "Aw, come'on 'Fonz. Come on, man," came the forlorn call from the Stead bench.

The Bullets, with 13-year-olds Rudolph McCullum and Billy Malone shooting well from outside, took the lead, 18-16, with three minutes left in the third quarter.

For a few minutes, the game seesawed. But Allen's shooting, passing and defense again began to dominate the other nine youngsters. Stead won, 36-25, with Allen scoring 17 points in the 20-minute game.

"He's gonna be a real good ballplayer," said Moore, "He's well beyond his age." With good coaching and some weight-training to fill him out, Allen could be terrific, Moore said. "He's gonna be 6-10."

Allen himself, in a postgame interview, predicted that he will only be "about 6-5." He said he lives on Vernon Street NW with his grandmother, his mother and his four brothers and sisters. A ninth grader at Francis Junior High School, Allen said this is his first year playing organized ball. He explained that he felt too awkward before.

He said he has been inspired to try to play professional basketball by watching his idols, such as Julius Erving of the Philadelphia 76ers and Larry Bird of the Boston Celtics. He has seen them on television, he said, but he has never been to a professional game because he hasn't had the money.

Allen said he estimated his chances of getting a scholarship to play college ball at 50 to 1, and of making the pros at 100 to 1. Maybe the odds are even longer, he added. His backup plan, he said, is to become a doctor. But he said that his low grades, mostly Cs and an F in English, might make it tough unless he improves his studies.

But then Allen said he is too young to worry too much about the future. He said it was time for him to get home. He walked out the gym door and headed home, wearing the sneakers with the "swoosh" on the side that he'd gotten as a gift from his mother.