The three-day, 284-mile Tour of America, the first professional bicycle road race in this country in more than 50 years, ended yesterday with 75 of the world's best cyclists bunched together in a wild, colorful 45 mph sprint along Constitution Avenue.

But the overall winner was not among the first to the finish line.

Slender, red-haired Bert Oosterbosch of the Netherlands clinched victory, just as he predicted when he got off a plane from Paris Thursday. He won by being the fastest in a 9.3 mile time trial held Saturday in the rain at Richmond National Battlefield Park.

Oosterbosch's T.I.-Raleigh team of Dutchmen also pedaled off with top team honors and the bulk of the $20,000 in team prizes. They stood triumphantly on the grandstand, spraying victory champagne on nearby members of the crowd of 35,000 that gathered around the Tidal Basin and Mall to watch the final laps.

The Tour of America attracted more than 200,000 spectators during its three-day ride over Virginia roads, even though few watchers could identify the riders or tell who was ahead.

Crowds lined the route, ignoring the mist and rain that plagued the first two days of the race, seemingly fascinated by the surging serpentine line of brightly colored racers. It made for a wild road race among close to 100 cars and motorcycles that drove two and three abreast down the road, horns honking, lights flashing and people hanging from car windows and sun roofs to assist cyclists, all the while shouting over CB radios in French, the primary language of the tour.

The final hour of the tour in downtown Washington contributed to traffic jams, with tourists coming to the race and to see the cherry blossoms.

In the final standings of the tour, Australian Phil Anderson placed second and Danish sprinter Dag Erik Pedersen placed third, both also because of fast solo time trials.

The first American to finish, Jonathan Boyer, was 24th, with John Patterson 26th and George Mount 32nd and amateur rider Bill Watkins, an Army captain from Fort Belvoir, was 39th.

The surprise of the race was the good showing by Canadian amateur riders, who placed fifth as a team and whose top rider, Steve Bauer, placed fifth in individual standings. Another Canadian, professional Ron Hayman on the Seven-11 team, won yesterday's stage of the tour in the final sprint on Constitution Avenue.

Many of the American riders were amateurs or recently turned professionals. But the race over flat terrain, with a time trial, was unlike the short races many U.S. riders have excelled in, or the long mountainous European tours in which Boyer, one of America's best all-round cyclists, does well.

With $100,000 in prizes, the richest purse ever in a bicycle road race, this first Tour of America became instantaneously popular with European racers, who struggled to get on one of the 15 teams.

Not all succeeded, including the most famous, Bernard Hinault, four-time winner of the Tour de France, after which the fledgling Tour of America is modeled.

Scrawled in chalk on Constitution Avenue, at the finish line, for tour racers to cycle over was: "Ou Etes- Vous, Hinault?"

Hinault had said he would ride in the tour, and as a very strong time trial rider might have won it. But French racing officials forced him at the last minute to ride in yesterday's Paris-Roubaix race, known as the "Hell of the North," which ended in a snowstorm as more than 200 of the 225 riders, including Hinault, dropped out.

Hinault flew over for the start of the Tour of America to promote it, and French officials have promised he will enter the second tour next spring. That race is expected to be a five- to seven-day race that will go into the mountains of Virginia, Maryland and Pennsylvania and end again in Washington.

Oosterbosch, a time-trial specialist, started the 1983 European racing season in February by winning a time trial and a similar three-day tour in southern France.

His 33-second edge over Anderson going into yesterday's final 76-mile leg, which began in a light mist in Fredericksburg, Va., may seem infinitesimal in a race that lasted 29 hours and 28 minutes.

But almost no riders could escape for long from the fast pack; the only successful breakaways were on Richmond's hills on Saturday. Anderson attempted a solo breakaway and won a sprint in Woodbridge on Rte. 1, one of two held during yesterday's race. It earned him a three-second bonus and reduced Oosterbosch's lead to 30 seconds, but that was how it ended.

As riders gathered last night at a hotel near the White House, preparing to take a charter flight back to France at midnight and taking a few final spins around Washington on their bikes, almost all apparently were delighted to have come.

For Oosterbosch and most cyclists, it was their first tour of America, a short three days that left them suffering from jet lag and with only a memory of wet Virginia roads and a fleeting sunny glimpse of cherry blossoms.

"But we liked it. We liked the smooth roads, the small number of riders, friendly crowds and the money," said Oosterbosch's teammate, Leo Van Vliet, who won the second stage in Richmond Saturday.

"In Europe, in the Tour de France, you have 170 riders on roads that aren't so good, and then the mountains. Here, my team, we especially liked the flat course," which for much of the 284 miles was almost as flat as Holland.