Saturday, 4 p.m., 86 degress.

The dark heads and bright clothes of 10,000 young Washingtonians on the field of RFK Stadium break into the fragments of a vast Impressionist painting. A simmering effect over the surface gradually resolves itself into the bobbing of groups of tightly packed dancers and the shaking of thousands of upraised arms. Driven by the insistence of the disco beat, couples bump hips, knees and elbows; on the edge of a giant stage platform, a roadie dances in exact tandem with a girl 10 feet below.

Another 20,000 spectators in the upper stands, where the contagion of bodily movement does not reach, slump down in their seats with their feet on the chairs in front, patting their legs in rhythm.

They are 16, 23, 26; only a few as old as 30. They wear blue jeans or cutofts, Danskinsor swimsuits, unknotted bow ties and guerrilla drabs with Castro caps. They smoke, cigarettes and a little herb and they give the other hip, young music-lovers a complete looking-over.

All had gathered for the "Chocolate Jam" and all-day celebration of music and black pride with the slogan. "One Nation Under a Groove." The first rock event at RFK in two years, it drew surprisingly fewer than the predicted 45,000-50,000: but higher-than-average ticket prices - $12 during the week and $14 at the stadium Saturday - offset the smaller sales.

The main attractions were Parliament-Funkadelic, the creators of Funk and its most "heavy metal" proponents, and Bootsy Collins, the bubblegum funker who aims his act at the preadolescent "chocolate bunnies."

At 8 p.m., with the show far behind schedule, the crowd began chanting, "Bootsy! Bootsy" while the sequined object of their adoration reclined in his limousine. "I betja he's waiting for it to get darker so his lights'll show up better," muttered a longtime concert security man.

At 9 o'clock, the concert's scheduled closing time, Bootsy was just getting off stage ("Bootzilla! Bootzilla!"), and it was obvious that an extension would be necessary. In fact, since the stage change for Funkadelic stretched out for over an hour, the Jam tvntually ran nearly 2 1/2 heeours overtime.

Throughout the day, stage changes consumed more time than performances, but the couples who had danced to the live disco of Raydio music now danced to the recorded music over the P.A. as the stagehands worked. A green-and-white beach umbrella, grasped at the pole by an enthusiastic dancer, spun hypnotically, song after song. Whistles blew, tambourines rang, maracas hissed. "Hey, man, where ar you?" an impatient teen-ager in a British racing cap snapped into the phone. "It's goin' down all day long!"

Outside the stadium, arriving fans had to pass through a gauntict of concessions trucks pulled up to the curbs, paralleled by card-table vendors hawking jewelry, sun visors and "Chocolate Jam" T-shirts. A few small clutches of picnickers remained outside, content with distorted snatches of music over the studium walls. Dismounted policemen leaned into the shadows of their horses.

Even as much sound as was generated by the gross of amplifiers was dissipated in the cavernous stadium: only the jets of National Airport and Andrews Ai Force Base passing silently overhead hinted at the volume level.

"I saw 'em at the Capital Centre," said a girl, snapping her fingers at Bootsy Collins. "He had on a better outfit. It was louder, too."