Arlington County officials posted "No Parking" signs yesterday and promised to tow away the cars of about 800 Metro subway riders who have been enjoying free parking at the doorstep of the Pentagon City station for the last six weeks.

Commuters returning from work yesterday afternoon expressed surprise and anger to find the signs interspersed among the cars parked on the rutted dirt lot around the station.

"I'm going back to driving my car downtown - to hell with it," snapped Mike Gordon, a 32-year-old financial consultant.

The ban on parking follows a July 30 County Board public hearing in which some residents near the station opposed the use of the site as a parking lot for fear it would congest traffic and add to air pollution.

The County Board voted 3 to 2 not to amend the county's zoning ordinance to allow a commercial parking lot on the site.

"My question is what is what are we congesting? There's nothing here," Gordon said, looking out over the sea of cars and the largely undeveloped land near the station, just across Interstate Rte. 395 south of the Pentagon.

"It's going to add to pollution even more when all those cars are driving into town," said Anita K. Scott, an Arlington County school teacher returning from the District. "If there's no parking lot, no one is going to take the Metro" from Pentagon City, she added.

About 1,500 people used the Pentagon City Station on Monday, according to Metro. Most of the these people parked their cars on the adjacent lot.

The number of people using the station is sure to decrease because of the parking ban, a Metro spokesman said.

"There's nothing Metro can do. We are at the mercy of the countries. They tell us what to do," he said. Arlington served Cafritz Co., the owner of the undeveloped land surrounding the Metro stop, with a notice yesterday advising it that it had 10 days in which to comply with the parking bann.

A Cafritz spokesman said the company would post signs saying "Private Property - No Parking." The spokesman said a ditch one to two feet deep will be dug around the lot this weekend and a berm erected to deter cars from entering the lot.

"We went along with the county on the (rejected) plan (to turn it into a commercial parking lot). And now we're confronted with a summons and the expense of doing this work - it's Catch-22," the Cafritz spokesman said.

The property will now go unused for an estimated year to two years until Rose Brothers, a New York construction company, begins work building a commercial-residential complex on the site, the Cafritz spokesman said.

One Arlington transportation official lamented the loss of the parking facility. "It seemed like such a perfect location for parking," he said.

County administration assistant Oliver Merriam suggested that those who had been parking on the lot could "take a bus" now and leave their cars at home.

But subway riders like Gordon don't like the suggestion.

He would have to pay $1.80 for a bus-subway round-trip fare from his home in Shirlington, while he can drive and park downtown in a lot for $3.10, he said.

"For $1.30, the convenience of having the car outweighs the agony of riding the bus," he said.