About 500 commuters defied an Arlington ban on parking at a dirt lot surrounding the Pentagon City Metro subway station yesterday and left their cars there. But workmen started digging ditches around the lot to insure that commuters cannot enter the lot this morning.

Last month the Arlington County Board voted 3 to 2 against granting a temporary permit allowing a parking lot for Metro commuters on the site, because neighboring residents said it would attract more traffic and pollution to their community.

Yesterday, however, the plan appeared to be back-firing. More than 100 commuters who chose not to park in the lot and risk having their cars ticketed or towed instead parked along streets and road shoulders in the neighborhoods that are trying to get rid of them.

As many as 800 commuters a day have parked on the lot, located just across Interstate Rte. 395 south of the Pentagon, since Metro service was started in Virginia on July 1. Some Pentagon City station commuters apparently drove to work yesterday yor traveled by bus to the subway.

County police said they would begin ticketing any cars today that are able to get into the lot.

On Monday about 1,500 persons used the Pentagon City station. A Metro official conceded that the number of commuters using the station is likely to decrease if they have no place to park and do not ride buses.

But Arlington officials said yesterday Metro's woes are not their problem, although they favor public transportation.

"We had two goals in conflict with each other," said Board Chairman Joseph S. Wholey, who favored a temporary lot on the site which is owned by the Cafritz CO. "We support public transportation and we support protecting residential neighborhoods." Wholey said the latter goal won out.

"Some of the people will still ride Metro by [taking the] bus, and some others will still drive and park in the neighborhoods," Wholey said.

He said "there wasn't much of a complaint" about traffic from residents near the station. "The board voted on what they supposed the neighborhoods' reaction would be."

But Board member Dorothy Grotos said the purpose of public transportation is to get commuters into the habit of using all of it - including buses - and not drive through residential neighborhoods to use only some of the services.

"The purpose of Metro was not to have cars come in to the area," Grotos said. "Once people got used to it, it would be a hard habit to break."

Ellen Bozman, who also voted against the parking lot, said the estimated 800 persons who parked in the lot should ride buses to get to the Metro station once the buses are rerouted.

"The question will be whether the bus on the other end will be convenient enough for people to get to," Bozman said.

Wholey said he and the county staff initiated the idea of allowing a parking lot on the land, whicis currently zoned or commercial and residential use. A zoning change would be necessary to build a parking lot there.

The lot was intended to be temporary until Metro lines extending further out into the Virginia suburbs are opened in about three years. Wholey said that when such new stations are opened, users of the Pentagon City station would probably catch Metro trains at the more distant points.

The land will now lie vacant for one to two years until Rose Brothers, a New York construction company, begins building a commercial-residential complex there, according to a Cafritz spokesman.

"The thing to be kept in mind is the justification for [the Metro station] is the anticipated high-density development of the site," said board member John Purdy, who also voted against granting a temporary use permit for the parking lot. "I didn't see the point of putting up a temporary lot and getting people used to using that part of Metro."

Walter Frankland, the other Board member who voted to grant the use permit, was unavailable for comment.