Washington unveiled her sweetest spring splendors yesterday for the celebrators of Earth Day '80. Crisp, sunny, the sky bluer than blue, the breezes bearing the perfume of ten thousand azaleas and tulips draped about the city -- it couldn't have been more perfect.

The first Earth Day was 10 years ago and there was an air of rancor and radicalism about it. But the atmosphere yesterday was low-key and friendly, an atmosphere of success for the environmental movement that has convinced America that it is critical to save "this infinitely beautiful little . . . planet called earth," in the words of Earth Day Chairman Byron Kennard at a dawn sunrise ceremony at the Jefferson Memorial.

Only a couple of hundred people showed up for that, but later in Lafayette Park in front of the White House more than 3,000 bicycle-riding commuters converged to hear Dennis Christopher, star of the Oscar-winning bicycle movie "Breaking Away," say that he had attended the first Earth Day in 1970 and believes the way to save energy is to "just keep pedaling and turning off the lights."

Thousands of people later in the day examined environmental exhibits on the Mall, attended films and seminars, and listened to speakers proclaim the need to continue the environmental fight. Most were tourists.

Elsewhere in the nation there were festivals and other observances. In New York, 10 blocks of Sixth Avenue were cordoned off for a street fair. In New London, Conn., a windmill was christened on the Connecticut College library. In Illinois, volunteers collected trash along 90 miles of highway between Champlain and Springfield.

While 10 to 20 million participated in Earth Day a decade ago, the organizers this time said they would be content with 3 million in various events across the country.

United Press International reported that more than 1,000 communities from coast to coast marked Earth Day yesterday with some sort of activity.

Among those biking to Lafayette Park in Washington was Secretary of Transportation Neil Goldschmidt, who borrowed a bike to come to the rally and announce a new DOT program to "make it easier and safer for more Americans to bicycle to work." The program increases the share of federal aid for highway projects that benefit bicycling.

Goldshcmidt said the program could put as many as 2.5 million commuters on bicycles by 1985, compared with 400,000 today. He said the program would save the nation up to 23.5 million barrels of oil a year -- about as much as the nation uses in one day.

But at least one bicycle commuter yesterday, Roger Drissel, found that the enthusiasm for bicycling may not have completely penetrated the ranks of the bureaucracy. Lacking a lock, Drissel parked his bike in his ninth-floor downtown General Services Administration office and was told by a guard to remove it from the building.

"It's typical of the kind of people that are running the country today," lamented Drissel, referring to the guard. "Their visions are limited. For one thing, they sit around and watch TV all the time. They aren't out bicycling. . . ."

Sen. Edmund S. Muskie (D-Maine) called yesterday for a national commitment to energy conservation to ensure that environmental ideas are not eclipsed by economic pressures and the quest for new energy sources.

"This new pressure, this new challenge to our environment calls for . . . an old spirit which must be born again," Muskie said in a statement.

He said 80 pieces of environmental legislation enacted during the past decade could be threatened now that polls show a plurality of Americans seem more concerned about energy and inflation than the environment.

On the Mall yesterday afternoon, schoolchildren ran around with a giant inflated whale as actor and environmentalist Eddie Albert declared from a podium, "We've got to save this earth. It's all we've got."

At the dawn ceremony, former secretary of the interior Stewart Udall read from Thoreau ("Every creature is better alive than dead."), Sen. Charles Percy (R-Ill.), in a jogging suit at 7 a.m. read from "Small Is Beautiful" ("Man is small . . ."), and a singer sang, "Whose garden was this: It must have been lovely."

Not all the bicyclists at the morning rally were commuters. Carol Northup, with her 3-year-old son Joseph in a jump seat, pedaled in from Alexandria to support Earth Day and because, Northrup said, "I had never been up this way before."

For the youngest bicyclist at the rally, Kimberley Cordray, 8, of Alexandria, pedaling into Washington was nothing new. "I did it last month with my father. His name is Bryan. He works at AID. We came in to go to church. And then we rode home again, too, 17 miles," Kimberley said proudly, holding a small, white bicycle helmet like her dad's.

"It was an easy ride. Next year I can ride to school on my bike. I'll be commuting too." She rode a small two-wheeler her father bought at a yard sale for $5. "It works fine," she said.