Fears that terrorists might bomb the Smithsonian Institution's Air and Space Museum by detonating bomb-filled cars in its underground garage prompted a decision to close the garage to visitors, museum officials said yesterday.

Starting today, the museum will divert visitors to the open parking lot beside the National Museum of Natural History at 10th Street and Constitution Avenue NW, which museum officials acknowledge will inconvenience some visitors and lessen the number of available public parking spaces.

The Smithsonian also has decided to bar the public from the partially enclosed parking lot at the National Museum of American History.

The substitute lot at the Natural History Museum is next door to the American History Museum, but is across the Mall and about six blocks from the Air and Space Museum at Seventh Street and Independence Avenue NW.

"It is our responsibility to our many visitors to take all reasonable steps that will assure their safety in the nation's capital," said Smithsonian Secretary Robert Adams. "Elementary prudence dictates that underground public parking in this location constitutes an unnecessary risk."

Smithsonian officials said the decision was not prompted by a specific terrorist threat, but had been recommended by independent consultants. It is part of a much wider program tightening security procedures at Smithsonian buildings used by the public, and was set in motion by the bombing of the Marine barracks in Beirut in October 1983, Smithsonian officials said.

The closing stems from a heightened concern about underground parking areas in government buildings, they said. Until yesterday, the Air and Space Museum was one of the only government structures in the Washington area with an underground lot open to the public, officials said.

Other government buildings owned or operated by the General Services Administration generally bar most visitors from their underground parking areas, and admit only staff members and some outsiders with permits, said GSA spokesman Dale Bruce.

Starting today, only museum staff members will be allowed to park in the underground garage at Air and Space and the partially enclosed lot at the American History Museum.

On weekends until today, 496 parking spaces were available to the public at Air and Space, and 78 spaces at American History. Because of the loss of these public spaces, and a planned increase in weekend public parking at Natural History from 226 to 320, there will be an overall decrease in public parking at the three museums on weekends from 800 to 320, officials said.

On weekdays, when the numbers of visitors are lower, there will be a decrease from 367 to 320 spaces at the three museums.

"The dramatic impact is on the weekends," said Smithsonian spokeswoman Madeleine Jacobs. "It's an inconvenience, but we don't think it will affect tourism. The best way to get to the Mall is still by public transportation."

Smithsonian officials said that all museum staff members with parking permits will be able to park at one or another of the lots, and that there might be a slight increase in staff parking. Parking is a major concern in the Mall area, and government employes guard their parking passes, doled out under a complex system according to seniority, grade level and whether one drives in a car pool.

Officials said that the Smithsonian expects to lose revenue because of the closing of the lots to the public. Visitors had paid $1.50 an hour for the first three hours, and $1.75 an hour after that. The average visitors' payment was $5.25, museum spokesmen said.

There are at least five commercial parking lots within a few blocks of the museums that are generally less expensive, museum officials said.

There is an irony to the Smithsonian's decision to close the Air and Space Museum's parking lot, because the institution has been trying to promote the convenience of its underground parking since the museum opened 10 years ago, on July 1, 1976. On many weekends, cars lined up around the block to get into the lot.

"We anticipate confusion," Jacobs said. "We're really sorry we have to do it."