More than 400 amateur and professional cyclists, including Olympic double-medalist Leonard Harvey Nitz and dozens of Washington's fastest bicycle messengers, are scheduled to compete for more than $10,000 in prizes today in the 17th annual National Capitol Open races on the Ellipse.

The flat, fast course around the President's Park, between the White House and the Washington Monument, favors sprinters and track cycling stars such as Nitz, who won the Open the last time he raced here in 1979. A silver and bronze medalist in track events at last summer's Olympics, Nitz has a record 13 national cycling titles.

Today's races also bring to Washington cyclists riding for more than half a dozen new commercially sponsored bike racing teams, most created following America's Olympic cycling success.

Most of the favored men and women are team-sponsored amateurs or professionals. And team tactics like blocking are expected to be a factor even on the flat, cornerless Ellipse, where successful breakaways from tight packs of up to 120 riders are difficult.

"Bike racing in this country is becoming a team sport, as it is in Europe, not an individual sport" as it has been for many years in the United States, said Mike Butler, past president and race team director for the National Capital Velo Club, which helped create the Open in 1969 and has organized it since.

"What you're going to see on Sunday are team races, tracky-type races and tactics . . . with guys going out and trying to break up the pack and lead their sprinters up through the pack," to put them in good positions for breakaways or the final sprint.

The featured 50-kilometer, 50-lap men's senior I and II race, at 2:30 p.m., will see a mix of amateur and professional riders. Favored riders include 1984 winner Matt Eaton, who also won the '84 Tour of England Milk Race; three-time Open winner Bruce Donaghy, who recently turned pro and is riding for a new Alfa-Romeo team; former U.S. National Team member Tom Prehn of Annapolis, and Paul Pearson, 30, a pro who is a graduate of Bethesda's Walter Johnson High School.

Pearson was one of the few American cyclists in the 275-mile Tour of America race, which brought the first large group of European racers to this country and ended in Washington during the 1983 Cherry Blossom Festival.

Nitz is riding for a powerful 7-Eleven team with Jeff Bradley, a 1984 Olympic alternate, and 1980 Olympian Tom Schuler.

The Open women's race regularly has seen some of the nation's rising cyclists, including Olympic silver-medalist Rebecca Twigg, who won here in 1982 but this week is on a 7-Eleven team in the Tour of Holland. Leading contenders include Ellen Braun, silver medalist in the 1984 national track cycling championships; Carol Addy, silver medalist in the 1984 criterium nationals, and Patti Cashman, 1984 national time trial champion.

Action starts at 9 a.m. with a new "midget" race for entry-level racers, boys and girls ages 9-11 licensed by the U.S. Cycling Federation, the governing body of amateur cycling that chooses U.S. National and Olympic cycling teams.

The only novice racers will be in the D.C. Messengers Challenge at 1:30 p.m., although to compete for $200 in prizes and a Golden Wheel Award even the messengers must have licenses -- D.C. courier licenses. The race, popular with spectators last year, has a Le Mans-type start with messengers running to a fence, unlocking bikes, doing two laps and locking bikes back up against the fence.

The Washington region has some of the nation's strongest older riders, "veterans" over 35 and "masters" over 45, entered in the 10 a.m. veterans race. Favorites include Jim Montgomery of Herndon, a five-time national veterans champion; Jerry Nugent of Bowie, national masters track pursuit champion the past three years; Paul Didier of Ashburn, Va., the current masters national criterium champion, and Rob Lea of Queen Anne's, Md., 1982 national veterans pursuit and kilo champion.