With the help of the first-day-of-summer sun beaming through a wispy, clouded sky, an estimated 20,000 persons marched along Constitution Avenue yesterday to begin the D.C. Rrecreation Department summer of festivities.

The theme for this year's parade, in the face of proposals by Mayor Marion Barry to close 11 recreation centers because of a municipal financial crisis was austerity.

Indeed, most parade floats were simple, homemade constructions, pulled along by pickup trucks and cars. They seemed to reflect the message spelled out by one in particular. That float, a plain old flatbed truck covered with a bit of carpet and hauling four smiling children, caried the explicit message that government officials are trying to get across:

"Do More With Less," it read.

The most elaborate float was fashioned in the likeness of a huge, gray whale that squirted water as it wagged its tail. "Have a whale of a summer." It read.

But the biggest attention getters were the children, who turned out in droves. This was the first weekend since school officially ended for the summer.

Nikola Smith, 4, looked a bit nervous as she was helped onto a blue and white plywood roller skate float mounted on a flatbed truck. "Get rolling for summer," it said.

Among the spectators, whose number was estimated by the U.S. Park Police at 5,000, was Dora Dozier, who came from her crowded apartment in Anacostia with her grandchildren.

Dozier beamed as the procession passed her way.There were military bands, Nikola's "roller skate" float, and a squadron of about 25 children on identical steel tricycles with big red wheels and curved handlebars.

"I just love it," Dozier said. "I was gonna go fishing but I had to make the parade. It's the way the little kids participate," she said.

Gilbert Webster, a day-care worker employed by the city, was a participant on the "roller skate float." Although he was enjoying himself yesterday, he still expressed concern for the future.

"They're trying to more or less preserve the programs for the youths, but if they get desperate, they'll get around to us, too," he said.

Several men from the Recreation Department's Roving Leaders program, counselors for "hostile and aggressive youth," enjoyed the parade beneath shady trees, but said they were concerned about the possible effect of cutbacks in their department.

Ed Smith, one of the leaders, watched as one group of children dressed in some of the 9,000 pairs of shorts and T-shirts donated for the parade highstepped by. "They lost their Roving Leader on June 6," he said.

Day-care teacher Linda Keyes kept the ranks of the tricycle brigade straight as she marched.

"There are a lot kids in the parade and that's the idea to get them to participate in something organized," she said, while waving to some adoring former pupils on the sidewalk.

For some of the observers, it was the cheerleaders from Barry Farms who provided the high point of the parade. The girls, with the backing of 15 drummers and a cowbell soloist, precociously dipped their hips "rocking the house," while the boys from the Turkey Thicket soccer team crooned and giggled.

"Right on," they cheered. "we like the girls from Ward 8."