Metro's first Saturday of regular scheduled subway service was a roaring success yesterday as thousands of Washingtonians and some tourists clambered aboard the trains and rode around town.

It was one of those glorious, early autumn days that rewards us for enduring August, and Metro's trains responded to the occasion by not missing a beat.

Although patronage had slackened sharply by 8 and 9 p.m., full trains - standing room full - were seen on both the Red and Blue lines during the early afternoon as people went shopping, stood on the National Airport platform to watch planes take off and land, rode the world's second-longest escalator at Rosslyn and took pictures of friends standing on platforms while trains came into stations.

"I love it," said Anne Early, who walked to the Farragut North station from her home near Scott Circle NW. "I'm going down to Gallery Place and I'll do some shopping. I'm so glad they're open on Saturday, because otherwise I'd have to walk or take the cab."

Metro's riders yesterday were a relaxed, happy crowd. Almost everybody was casually dressed and hundreds of families were using the day to take the kids on the train. There were teen-agers, young lovers, old folks, blacks and whites.

School-age children with D.C. public library books under their arms stood transfixed on the National Airport platform to watch the best aviation show in town. Others wearing Annandale T-shirts wandered around outside the D.C. Armory.

Crowds thronged in and out of the Smithsonian station on the Mall from early in the morning. Hundreds of people - up to 500 an hour by midafternoon - walked into the Woodward/Lothrop department store directly from the Metro Center station.

"It's like rush hour all the time," said Ameilia McFadden, a Woodies employe who was counting potential customers. A Woodies spokesman said that the Saturday hours were clearly helping the downtown store, but he pointed out that a major sale also was in progress.

At Crystal Underground next to the Metro subway station in Crystal City, there was business on Saturday for a change. "This is definitely a better crowd, maybe 15 to 20 percent better," said Steve McDougal, manager of the House of Music, a record and tape store. "Some Saturdays we just watched the air go by."

In Silver Spring, Annie Laurie boarded a Metro train for the first time in her life. She was headed for National Airport and was lugging a large suitcase and a smaller bag.

"I thought since I'd be gone about a week I'd give (Metro) a try instead of parking car at the airport," she said. As the train whooshed downtown, she turned and said. "You know, this is not bad." She drives to work weekdays. She might now try commuting by Metro, she said.

There were a large number of first-time Metro users yesterday, and that meant the Metro's Achilles' heel - the Farecard system - was especially susceptible to glitches.

A station attendant at Eastern Market said, "It looks like this Saturday thing is going to catch on - if I could just get those machines to work." He waved in the direction of three Farecard vendors, two of which displayed the familiar "out of service" lights. A man with an armload of produce obviously just purchased at the market juggled bags to find change.

At Rhode Island Avenue, station attendant Jim Holcomb described business as typical of midday, then bustled over to help two people who could not get the faregates to accept freshly purchased Farecards.

But nobody seemed to mind. It was Saturday.

Harry Barley, a former Metro financial planner who decided to switch to rail operations was announcing over the public address system that people headed for the Air and Space Museum should exit on Independence Avenue; those going elsewhere should use the Mall.

It is one of Metro's best-kept secrets that the closest station by three blocks to both the Air and Space Museum and the Hirshhorn Museum is the L'Efant Plaza station. The Smithsonian station is best for History and Technology, the Freer Gallery and the Old Castle.

Nicholas J. Roll, Metro's assistant general manager for transit services, was so pleased with things yesterday he could hardly stand it. "It's just like a holiday out there," he said. "In the last week we've come from being a commuter railroad to a subway transportation system."

In the last week, Metro has inaugurated week night and Saturday service until midnight.Sunday service has been proposed for next September.

Metro's next big test will come Monday night after the Redskins-Dallas football game at RFK Stadium. Metro is planning to keep the subway open past midnight if necessary to get fans home and will have several extra trains ready at the Stadium-Armory station to handle the crowd.

Deborah Brown, who lives in Fairfax Village in Southeast Washington, complained about that yesterday. "If they can wait for the Redskins," she said, "why can't the buses wait for me." She was transferring from the subway to a bus at Stadium-Armory.

At Rosslyn yesterday, where the escalator is 210 feet, 8 inches long and takes 1 minute, 45 seconds to ride from bottom to top, a mother and her 4-year-old, amusement-park wise son stood at the bottom and stared upward.

Mother: "Do you want to ride it?"

Son: "Mommy, can't we ride the little one?"

Mother: "There isn't a little one."

They rode it.