LAS VEGAS, Nev. (AP) _ One of America's worst kept military secrets, the F-117A stealth fighter, was unveiled Saturday to a crowd of thousands who cheered a demonstration flight by two of the multimillion dollar planes.

The two bat-shaped planes swept in under thin cloud cover and passed over Nellis Air Force Base, banked sharply against the Las Vegas skyline and landed to give the public its first closeup of the once top-secret aircraft.

There was an eerie whine as the lead craft, piloted by Capt. Randall Peterson of Rock Island, Ill., made a pass 58 feet above the runway. The second, piloted by Maj. Steven Charles of Springfield, Mass., then swept by the crowd, circled and landed.

Both jets were surprisingly quiet in flight and while taxiing to an area encircled by several thousand people, including military families, VIPs and 225 reporters and photographers, including 37 from five foreign countries.

Viewing was opened to the public later and as many as 150,000 people were expected.

Foreign journalists represented Britain, France, Australia, Japan and West Germany. Asked if any Soviet reporters were on hand, Tech. Sgt. Bobby Shelton, the stealth unit information officer, replied, ''They didn't ask us.''

The F-117A costs more than $100 million a copy, according to a recent report by the General Accounting Office. The Air Force gave a figure of $42.6 million - the so-called fly-away cost of one plane, without counting development costs for the program.

No performance details were released, other than that the radar-evading F- 117A is a single-seat, twin-engine subsonic jet. ''It is a fighter in every respect,'' Peterson, 30, told the crowd.

He said the jet has outstanding handling and gives the United States unprecedented capabilities. Pressed about those capabilities, he said, ''We're not allowed to discuss any capability. A lot of the information is still classified.''

Referring to an F-117A nickname that has surfaced, he said, ''Nobody that flies the stealth aircraft calls it a wobbly goblin.''

Also off-limits were questions on reports that a stealth fighter used in the U.S. invasion of Panama last year dropped bombs far from its target.

''The bombs hit exactly where they were aimed,'' Randall said before another Air Force officer broke in and said, ''We can't discuss information like that.''

''I only wish I could tell you what this fighter can really do,'' said Ben Rich, vice president of Lockheed Corp., which builds the jet at its secret ''Skunk Works'' in Burbank, Calif. ''The performance is awesome and the weapons system is unmatched anywhere in the world.''

Though the ''fighter'' designation suggests dogfights with other planes, the F-117A's main mission is as an attack jet. It is designed to sneak through enemy defenses and bomb selected targets such as defensive radar installations. The Air Force only recently lifted the shroud of secrecy on the F-117A, which long had been rumored to exist and has been flying for a decade. Previously, only pictures had been shown.

The stealth fighters are attached to the 37th Tactical Fighter Wing at the Tonopah Test Range, a remote airfield 140 miles northwest of Las Vegas.

Most of the 2,500 military personnel assigned to the program live in the Las Vegas area and commute to Tonopah each week. For their families, the public demonstration was the first view of the plane.

The F-117A is a relatively simple aircraft to fly and operate. said Col. Anthony Tolin, commander of the 37th. With only three accidents in nine years, it has the best safety record of any Air Force plane, he said.

A test pilot survived the crash of the initial plane. Two Air Force officers were killed in other accidents during operational training.

A total of 59 stealth fighters have been purchased and the last will be delivered this fall. Each costs $42.6 million.

Lt. Gen. Peter Kempf, commander of the 12th Air Force, said it took 31 months to develop the stealth fighter, compared to a normal aircraft development time of 10 to 12 years. The first stealth fighter flight was in 1981, and the first production aircraft were delivered to the Air Force a year later.

The plane was flown strictly at night and over remote deserts until November 1988, when the Air Force acknowledged the aircraft's existence.

Officials said the secrecy was lifted because the plane needed to start flying in daylight. Tonopah residents have reported seeing the F-117A in flight for several years.