They drove hundreds of weekend music fans wild in 19th Street yesterday afternoon. They blocked pedestrian traffic, encouraged streetwalkers, attracted swingers and spread the blues in the 99-degree heat.

The cause of all this chaos? "Widespread Depression." A band. Nine New York musicians invited to get both sides of the street workin' by Kramer Books and Afterwords Cafe. They did their job.

Perched on black cafe chairs, parked by the curbs, lounging on the grass of the Dupont Plaza Hotel and just standing around, the weekend faithful off the Dupont Circle neighborhood and plenty of other people turned up for a "Dancin' in the Streets" party thrown by the cafe between 2 and 6 p.m.

The Rolling Rock beer flowed at $1 a shot, and sandwiches of roast beef, turkey, sausage and cheese disappeared fast from the serving table on the Kramer patio. Kids on roller skates put on their brakes. Joggers jogged in place. Book browsers emerged into the sun.

"It's a party for our customers who come in all year," explained Mark Lindgren, a Kramer's manager vaguely visible behind his sunglasses and drooping Panama hat.

The dancing began not long after the first notes of an old Basie tune called "Topsy" started wiping out nearby motor rumble. Jeff Weiss of the Cuban-Haitian task force and Wendy Schpero, legal secretary at Brustein and Manaseuit, led the way, taking to the tar with what Weiss described as "a modified lindy boogie."

Many of the folks who didn't show at Ocean City or Rehoboth soaked up the sun and the blues at the same time. Some workers relaxed, like the maids who peered out between the curtains of a few of the hotel's rooms. Others struggled to get things done.

Jerry Barrish, a bearded 39-year-old determined to keep his street chair between sets despite the dubious oxygen content outdoors, balanced a healthy tone entitled "Metropolis and Region" on his lap. He's writing an M.A. thesis on the growth of cities for the sociology department of the University of Washington, Seattle, and figured the commotion was part of his work day.

"Keeps you off the streets -- on the streets that is," Barrish, tooling his phrase.

Al Kettler leaned back against a store front, head bent toward his pad, working on a group portrait.

"I just draw all the time," Kettler shrugged. "But I don't draw people very well. I came to draw people."

Steve Schultz, a law clerk, read a day-old newspaper. Lorraine Rose, a 27-year old artist/stress manager -- she's not sure which career should come first -- said she had left her husband on their honeymoon. He was with all the relatives, celebrating their secret month-old marriage, while she listened to the jazz.

Since the cafe hadn't obtained a permit to serve liquor on the street, Kramer's employes did their best to keep customers with drinks from going beyond the cafe confines to the street. The hope was that everyone would behave as well as the fellow with the T-shirt reading: "I have no problem. I drink. I get drunk. I fall down. No problem."

Two hours into the jazz, Kramer had lots of customers and no problems. That permitted people like Lonnie Wilfong, a 63-year-old retiree from the U.S. Army band who had come in from McLean, to enjoy peacefully the band's rearrangements of classic Basie, Ellington, Lunceford and Hines tunes.

"I heard a few cuts from one of their albums and I was impressed," Wilfong commented, nodding to his friend Carmen Campagnoli, 62, who runs his own little band called "Music by Carmen" and likes to keep in touch.

Manager and vocalist Jon Holtzman, who sounds a lot like Sinatra used to, did a brisk business between sets selling copies of the band's two albums. One of them, "Downtown Uproar," won a High Fidelity award as best traditional jazz album of 1979.

With all the business coming to Kramer's outside patio, co-owner David Tenney got caught up in the excitement and danced in the street himself, chucking his sandals to do a barefoot boogaloo for a few sizzling seconds. His blisters should be gone in plenty of time for next year's promised merriment.