The Fairfax County Board of Supervisors last night blocked plans for two mammoth office complexes that highway officials said would have added thousands more cars daily to the area's already choked roads.

The supervisors voted 8 to 0 with one abstention against changing a master plan to permit the development on the 350-acre Chiles tract on both sides of Rte. 50 just east of the Capital Belway.

Wills and Plank, a Vienna firm, and Western Development Corp. of Washington wanted to build 2.4 million square feet of office space on the land. Their proposal drew strong opposition from citizen associations and the Virginia Department of Highways and Transportation.

The supervisors' decision was called "an important precedent" by Ken Wright, a member of an association of citizen groups called SCORE (for Support and Conserve Our Residential Environment).

"In the past, economic development was the basis for all decisions. This time the board listened to the people. They put environmental considerations first," said Wright, who lives in Broyhill Park and is an official of the Environmental Protection Agency.

Another point of view was advanced by Supervisor Audrey More (D-Annandale), who said, "This shows what can happen in the election year. This wouldn't have happened otherwise." All the seats on the board of supervisors are up for election this November.

Citizens from SCORE mounted an elaborate lobbying effort to defeat the proposal. The organization assigned groups of citizens to present their case to each of the nine supervisors.

Additional significant opposition came from the state highway department, which told the supervisors that any development of more than 750,000 square feet would choke the beltway Rte. 50 intersection. Rte. 50, a fourlane highway, already handles 50,000 vehicles a day; the highway department estimated that the proposed development would add 24,000 cars a day.

On the county's master plan the land is now set aside for housing. In the northeast tract, developers still can apply for up to 750,000 square feet of office space.