Vladilnir Makarkin of the Soviet Union outkicked a pack of 20 challengers in a heart-throbbing finishing sprint to win the 76-mile Junior World Cycling Championship road race by the length of his front wheel yesterday in Rock Creek.

In a 40-plus mph closing pack so tight that the first 14 finishers were credited with the same time, East Germany's Hubert Denstedt placed second. He was followed by Thomas Landis of Switzerland and the Soviet Union's Alguiss Vritkus in a dead heat for third.

Greg LeMond of Reno, Nev., who turns 17 today, placed ninth, the best showing by an American in the six-year history of the event. LeMond, the national junior champion, still will be eligible for next year's junior championship.

Two other Americans from the six starters placed in the top 20: Jeff Bradley of Davenport, Iowa, ended up 16th and Denver's Ron Kiefel 19th. Thursday Rogers of Los Angeles placed 44th.

Switzerland won team bragging rights, placing four riders in the top 14, followed by the Soviet Union and West Germany with three each and East Germany with two.

Ninety of 133 starters completed the 10-lap race. Three riders were treated and released at Sibley Hospital for minor abrasions suffered during spills. Greg Demgen of La Crosse, Wis., who had led the race for part of lap seven, was forced out when his bike seat broke.

Denmark's Tom Peterson broke away from the pack on the first lap and completed it 13.3 seconds ahead of the next group of riders. But by lap two Belgium's Ronny Penxten and Holland's Wilhelm Bouman were close behind in second and third, respectively. At the end of lap five Italy's Raniero Gradi led with a 12-second margin.

The main pack closed the gap completely on lap six and did not allow another breakaway until the ninth circuit. By the middle of the ninth, Thomas Barth of East Germany and Holland's Jacobus Hanegraaf, riding one-two, had taken a 55-second lead over the pack. The pair entered the final lap 50 seconds up.

But then the duo fell victim to the pack's closing spint on the backstretch, especially Grant Road, a tortuous 2,000-foot hill. By the top of the hill the pack had made up all but 10 seconds of the pair's margin.

The pack continued to close and caught up at the top of the Oregon Avenue hill at the intersection of Wise Road. Whizzing around the final curve of the stretch along Beach Drive, the brightly-dressed riders resembled a flying rainbow, as they finished to the wild cheers of thousands lining the course.

Soviet Union Coach Germady Gorunov said Makarkin, a native of Tashkent in southern Russia, had ridden the race as planned. "I wanted him to stay with the team, drafting the others to save his strength for the final sprint," Gorunov said. "Even though he is not our best sprinter, he is our best finisher." Makarkin, who will turn 18 in July, is the first Russian to win the event.

The race took 2 hours 55 minutes 10 seconds, an average of better than 25 mph. It was the sixth of seven events in the World Championships with Wednesday's team time trials along the Maryland section of George Washington Parkway (10 a.m.) wrapping things up.

The Soviet Union has now won three gold medals and two silvers. East Germany has collected two golds, two silvers and a bronze.

Gorunov would not discuss the turnaround his team made from last year's championships, when they were shut out of gold medals. He only admitted the team had used a "special compound to prevent cramps."

Another member of the jubilant Soviet coaching staff said that yesterday's race required "the health of an elephant and the trickiness of an ape."