There were cars parked illegally in front of entrances, next to curbs, in gutters, in the middle of fire lanes and on top of sidewalks.

While jets rumbled off smoothly on schedule yesterday at National Airport, it was near-gridlock in the terminal's parking lots.

For reasons unknown, virtually every one of the airport's 4,200 legal parking spaces was filled by late morning, prompting Federal Aviation Administration officials to issue pleas over local radio stations for people to take Metro, not their cars, to the airport.

"We think people are starting their Thanksgiving vacations early to take advantage of the low airline discount fares," said Hugh Riddle Jr., deputy director of both National and Dulles airports. "But we really don't know the reason."

A decision to open 400 of the 3,000 spaces reserved for airport employes to the public eased the problem for some frustrated travelers. The employe parking was made available free of charge. But the out-of-the-way spaces meant long hikes -- with luggage -- back to the terminal for many.

"I have nearly missed my plane," groused Jim Spencer of Northeast Washington, who was scheduled to fly to Tallahassee, Fla., on business, and chose to leave from National because it seemed convenient. "I should have left from Balitmore."

Spencer was working up a healthy sweat despite the biting, 40-degree weather as he trudged from his car, which he left in an employe lot on the far side of the airport grounds, to the terminal after a half-hour search for a space.

Some travelers said they circled the entire parking area before spotting guides who directed them into a space.

Airport officials said the overflow problem began to build on Tuesday as helpless authorities looked on. "We had more people use the airport last year and yet we didn't have this kind of problem," said the puzzled Riddle. "Maybe we'll get the answer in a few months when we analyze the parking data." "

Riddle said National's parking problems are similar to those of other major airports. More than 15 million passengers uses the airport in 1979, compared to the estimated 14.7 million expected to use it this year. Metro officials estimate that one of every five people in the Washington are already use mass transit when traveling to National.

According to Riddle, some of the parking problem should be eliminated next month when 150 new spaces are opened. By next spring an additional 125 spaces will be available under a $350,000 parking expansion program.

The only parking area that appeared unaffected by yesterday's mad scramble for space was the lot reserved for Supreme Court justices, members of Congress and diplomats. While commuters were directed to the far ends of the airport to park their cars, dozens of spaces stood empty within shouting distance of the main terminal.

Asked if those spaces would ever be opened to the tax paying public, Riddle provided a one-word answer: "No."