For a day, just for a day, the underused Dulles International Airport may have been the busiest terminal in the country. Just as in the travel agent's dreams, tens of thousands of people -- perhaps as many as 150,000 -- squeezed into the terminal and spilled out onto the tarmac. It could have been O'Hare in Chicago, or the Dallas-Fort Worth International Airport.

The crowds will not last; yesterday was special. The occasion was Dulles' 20th anniversay bash and the airport threw the party for itself.

The turnout yesterday, far higher than any of the organizers expected, snarled mid-afternoon traffic on the Dulles access road for more than two miles. It brought the estimated weekend attendance at the birthday party to a quarter million.

Dulles Expo '82, as the fete was dubbed, was judged a success by all its organizers, chiefly the Loudoun County Chamber of Commerce. Yet despite the huge crowds, despite the parachuting British Royal Marines, the stunt planes, the B17 Flying Fortress, the endless exhibits and the SST Concorde that buzzed the runway in front of the observation booth before turning off toward Europe, many were doubtful about Dulles' future.

"Dulles won't work because of the Congressmen who want to keep all the flights at National," said a Vienna bicycle store owner. Said Dean A. Beasom of Burke: "I would use Dulles if the flights I wanted flew out of here."

If the crowds were skeptical, they were also entertained. Dulles was an airport-turned-carnival for the weekend.

That made for some rather odd sights, such as a tuxedoed string quartet playing stately classical music in the midst of a teeming main terminal building.

In back of the terminal, thousands of people roamed on the tarmac among the various planes set up there for public exibition. There were planes from the Second World War, small private planes, and the monster C5A transport plane, used by the military to carry helicopters and supplies.

For Kevin B. Jones, 9, of Manassas, the most exciting moment of the day was boarding the Federal Express craft. Kevin had never been inside an airplane, but he knew all about Federal Express: "Those commercials where the guy talks so fast."

The crowds confused the small contingent of people who were actually at the airport to travel. The men were easy to spot: unlike those in the sun-sapped crowd, they wore shirts and carried suitcases. They also appeared altogether puzzled.

"I fly in and out of here from Oklahoma City," said Kenneth M. Ferry. "I walked in here today and said, 'Now what in the dickens is this? It's never this crowded here.' "

Only Dale G. Jungst, a mathematics professor from Dekalb, Ill., was unruffled. "I'm used to a busy airport in Chicago," he said.