The Fairfax County Board of Supervisors last night approved construction of two high-rise towers in Tysons Corner that nearby residents had opposed at an emotional public hearing last month.

The towers, to be built on a hill just north of the intersection of Leesburg Pike and the Capital Beltway, will rise higher than any existing building in the county because of its location near the county's highest elevation. Neighbors said the towers might set a precedent that would transform Tysons Corner into an area like Rosslyn or Crystal City where there is intensive high-rise development.

The supervisors unanimously approved the $100 million complex after the developers agreed to build 17 stories instead of 20 as originally proposed. The developers, the Vienna law firm of Lewis, Mitchell & Moore and a partnership of other lawyers, also promised to preserve nearly half their 11.7-acre site as open space.

The site currently holds the Hollingswood subdivision, once a peaceful community of single-family homes that is now surrounded by the Tysons Marriott Hotel, the Tysons Corner regional shopping mall and other commercial development.

The developers touted their towers as "a significantly powerful image" in a letter to the county last fall. "The Tysons Corner area needs a landmark. . . ," attorney Robert M. Fitzgerald wrote.

Residents of nearby subdivisions, however, argued that the towers would loom over their backyards and reduce property values. The reduction to 17 stories satisfied two major nearby homeowners associations, but several citizens spoke against the project last night.