With more than 6,000 sets of credentials issued to news organizations worldwide, the first formal U.S. briefing yesterday on this week's summit drew about 200 journalists, most of them regular diplomatic correspondents based here.

This was to be the summit overture, as played by the U.S. side, after almost a week of Soviet experts briefing journalists. In fact, however, the briefing at the Commerce Department seemed more like a trial run for the media facilities than a prelude to the peace talks.

The real test may not have been of the telephones, the heating or the television hookups, all of which seemed to work. It was whether journalists from around the world could find their way from the working-press center at Commerce to the briefing auditorium.

Navigating the way through the corridors to the room where White House spokesman Marlin Fitzwater opened an on-the-record session with U.S. arms-control specialists would have challenged even the most adept Eagle Scout.

So, U.S. Information Agency officials taped a yellow line along the floors to reduce the number of journalists disappearing in the maze of hallways. Like a strange echo from the "Wizard of Oz," security guards kept advising journalists to "follow the yellow tape line."

On Monday through Thursday, the J.W. Marriott Hotel at 14th Street and Pennsylvania Avenue, will be the site of another international press center where White House and Soviet spokesmen are to brief reporters twice daily.

Until 11 p.m. tonight, though, the massive grand ballroom -- 100 yards long -- is to be filled with guests from other functions.

At a session Friday afternoon, USIA and Marriott officials tried to negotiate times for bringing in 600 extra telephone lines before the media room opens at 9 a.m. Monday. As the officials talked, a telephone technician rushed from the room and said to a passer-by: "Nine hours? Nine hours? This usually takes a week."

The invasion of journalists and their technical teams has made hotel denizens nervous, and hotel officials acknowledged last week that they know this will be a lot different than a bankers' convention.

"The Marriott people said they wanted us to assure them that the journalists were going to be well-behaved," said Robert Garrity, director of the Foreign Press Center. "I finally just said no way."

Burnett, Garrity and others said Marriott and White House officials worked out use of the hotel for the summit. "They know it is an extremely historic occasion and want to participate in it," Garrity said.

For each media credential, Marriott and the USIA have provided a package including reprinted Reagan speeches, a USIA briefing on Soviet jamming of U.S. government broadcasts into the Soviet Union and a T-shirt bearing the nation's flags, the date of the summit and hotel name.

Although media credentials will be as treasured as tickets on the 50-yard line, the passes have drawn criticism from U.S. journalists because of the stars and stripes.

The pass bears the red background with yellow hammer and sickle on the Soviet flag, merged with a version of the U.S. flag with three stars and seven stripes.

"What happened to the other 47 states?" one reporter asked, adding that it isn't something worth an official protest, at least not yet.