A caravan of young businessmen, locked arm in arm, scream like children as they roller skate past Gusti's Restaurant at 19th and M streets NW, headed for beer at the nearby LA Cafe bar and disco.

Carefree couples dining on the patio at Gusti's smile as they sit comfortably cool in ice-cream colored outfits, their summer tans glistening in the sultry night.

Pete Marchitelli, Gusti's night manager, watches all in amazement.

"I don't know where all of these people come from," he says. "Here it is nearly midnight and it looks like noon."

Roughly 5,000 people now hang out around 19th and M streets each night, according to most estimates, making it Washington's busiest street corner after dark.

With open air discos, terrace restaurants, theaters and boutiques, the "new downtown" has replaced Georgetown as the intown spot for city street life.

"Georgetown was popular when they had hippies up there," said Vincent Liano, a Gusti's employe. "The war protesters made Georgetown a happening place. Now we don't have any wars. The flower children have mellowed. They want to skate now."

For many years, this was a nondescript part of town with homes, ethnic delicatessens, ice houses, greasy sppon restaurants and small car dealerships. When the last families moved out around 10 years ago, massive office buildings began to go up.

"I used to wonder where they were going to find people to put in those buildings," Marchitelli said. "I understand now that New York is moving to Washington. That won't hurt us at all."

By and large, the supporters of the new downtown are tourists and young adult professionals who work in the area by day and return to socialize at night - that is, if they go home at all.

Although the numbers of persons enjoying the nightlife in Washington has been growing for about seven years, particularly since the Bicentennial, the Summer of 79 appears to be a record year.

Coinciding with the unprecedented redevelopment of the area is a growing number in Washington of people between the ages of 25 to 35, for whom most of the M Street nightlife appears to be geared.

"We just don't see teen-agers anymore," Liano says.

Mark Johnson, who has been selling balloons outside of Gusti's for over 10 years, agreed.

"There are a lot of people out here, but nobody is buying anything," Johnson said sadly. "I don't know where the children are. People just want to skate."

Although there are married couples around 19th and M at night, the new downtown has emerged as the playground of the swinging single, "This is the action center of the city," said Dennis Bolinar, who lives in Anchoage, Alaska, but heard about 19th Street from a cab driver at National Airport. His new friend, Laura Adack, from Philadelphia, whom he met in the Appletree Disco, agreed.

"Lots of men," she said.

As a carload of women arrived at 19th and M streets, Friday night, a group of handsome gay men donned their roller skates, ignoring them.

"It's not going to be like this, is it?" one woman asked her friend. "Just be patient," she was advised.

It used to be that a family would come in for dinner and stay until 2 a.m.," Marchitelli said. "Now dinner is over by 10:30 and they go skating or dancing. It's a different crowd from 10 years ago."

Rick Hershman, who uses Washington Bullets ("Bullettes") cheerleaders to promote his Hot Skates rental at the corner of 19th and M, says that he finds his clients "are just out for a good time." People come down here and see people skating and it seems like fun. It makes people smile."

Amy Robinson Goldson, a lawyer who has an office on M street, returned Friday night to skate around the area where she works. "After a hard day's work I have to release my tensions. The fresh air is rejuvenating. This area was about to pop anyway," she said. "Skating just made it happen."

Bob Devlin, a self-described "street singer" who plays country music while young girls drop dollars in a bucket before him, is also enthusiastic about the new downtown.

"The crowds are good. The people are good. Good vibes here," he says, peering at his bucket of money. "I'm just a street singer. I like singing for the people, and as you can see, here they are."

By early morning, they are still there, skaters, mainly, line the streets in silken gym shorts and breezy tank tops, damp with sweat. The body language is fierce.

"It's against the law to skate in the street," a policeman told one woman early yesterday morning. The woman, wearing hot pants and a loose blouse, whirled over to the patrol car window, causing the officers to blush when she leaned over inside the car.

"Please, Miss," the officer began. He paused.

"Please, what?" the sassy young lady wanted to know.

"Please be nice and don't skate in the street. Just last night a guy fell on a car and broke his wrist."

As the woman sashayed away, the officer raised two fingers in a victory sign. "The best beat in town," he exclaimed. CAPTION: Picture 1, A crowd gathers around Bob Devlin, left, who sings country songs for donations at 19th and M streets NW.; Picture 2, Mark Johnson, who sells balloons nearby, says the teen-agers are gone and few people buy balloons. Photos by John McDonnell - The Washington Post