WASHINGTON (AP) _ The Beach Boys wound up playing second fiddle to a dazzling fireworks display in a sudsy Fourth of July celebration on the Mall that drew more than a half-million revelers.

The free concert was to have ended before the fireworks started, but Mike Love and his sidekicks arrived late from an earlier gig in Philadelphia and took the stage more than an hour behind schedule.

Ten minutes into the Beach Boys' set of ''California Girls,'' ''Rhonda'' and other standards from the 1960s, the first rocket boomed over the Reflecting Pool.

Fireworks impressario George R. Zambelli kept the skies ablaze over Washington for nearly half an hour while the music played on.

Even Mr. T, the semi-bald strongman actor, was no match for the pyrotechnics as he banged drums for the Beach Boys. The bandstand on the Washington Monument grounds was only a few hundred yards from where Zambelli's crew fired their 4,800-shot, $29,000 barrage.

The National Park Service estimated the crowd at 550,000 - the biggest on the Mall since the Bicentennial. Earl Kittleman, the Park Service spokesman, said 72 people were arrested, mostly for intoxication or disorderly conduct. More than 1,100 were treated at first aid stations, chiefly for heat exhaustion and minor cuts.

Forty-two were taken to area hospitals, including one man flown out by helicopter after an apparent heart attack.

President and Mrs. Reagan watched part of the fireworks from the balcony of the White House, where they spent a quiet holiday and dined with friends.

Mrs. Reagan, who will turn 64 Saturday, was serenaded with a chorus of ''Happy Birthday, Dear Nancy,'' by White House staffers and their families on the South Lawn.

Many in the sun-baked crowd of young people on the Mall spent all day waiting for the concert. The Four Tops and other groups played into the early evening, but the crowd had to endure a 90-minute delay before Joan Jett and the Blackhearts and the Oak Ridge Boys warmed them up for the Beach Boys.

Love, the Beach Boys' lead singer, apologized for being late and said, ''The Park Service has asked us to end this show as quickly and quietly as possible.'' But after imploring the crowd to ''be nice to each other ... so we can get together every year like this,'' Love and the Beach Boys played for nearly 40 minutes.

While the area around the Washington Monument was strictly for a beer keg and cooler crowd, the wine-and-cheese set was picnicking outside the Capitol a mile away and enjoying a National Symphony concert, with Leonard Bernstein conducting his music from ''West Side Story.''

Zambelli said his Newcastle, Pa., company was putting on 1,400 shows across the nation on and around the Fourth, but the one on the Mall is ''the number one display ... The pride, the prestige, the recognition is here.''

And with a nod toward the rock music blaring behind him, he said, ''They're all here, really, for the fireworks as we all know.''

Tom Walsh, 21, came from his home in Livingston, N.J. ''I didn't even know the Beach Boys were playing,'' said Walsh, a Union College student who spent part of the afternoon juggling five balls.

Daniel Smith, 23, of Wauwatosa, Wis., was there with his bagpipes, kilts, and a 60-pound backpack he is carrying on a trek across America to raise money for cancer research. Smith has raised $5,000 since setting out from Maine in April.

The Beach Boys had originally planned a ''sea-to-shining-sea July 4 tour'' with further stops in Dallas and San Francisco. But the Dallas show was cancelled and San Francisco postponed, turning it into a trek from the Delaware River to the banks of the Potomac.