Fairfax County Republicans have given Ronald Reagan the bulk of their support, delivering the former California governor about 60 percent of the delegates selected for upcoming district and state GOP conventions.

Nick Panuzio, chairman of the county's Republican committee, said yesterday that Reagan supporters captured the largest number of the nearly 1,800 delegates chosen at eight mass meetings in the county Tuesday night. The closest challenger was George Bush, who received backing from about 35 percent of the delegates selected.

Illinois Rep. John Anderson picked up the remaining delegate support from among the 5,000 county Republicans who attended the meetings.

The delegates won't actually cast votes for their presidential favorite until the GOP's 8th and 10th congressional district conventions on May 17.

All those chosen at the mass meetings, however, will attend both the district conventions and the state convention in Richmond June 6-7 and will help decide how Virginia's 51 delegate votes at the national convention will be apportioned among the candidates.

Though Reagan won the most delegate votes in Northern Virginia's two congressional districts four years ago, Panuzio said this year's race in the 10th District should be closer.

Reagan, who had the overwhelming support of Virginia Republicans at the 1976 national conventional that nominated then-president Gerald Ford, is expected to be the big winner in the state again this year. Party moderates and liberals, particularly in Northern Virginia, are expected to make their presence felt.

Alexandria Republicans already have expressed a preference for Bush. Arlington, Fairfax City and Prince William County have yet to hold their meetings.

"A lot depends on Prince William," said Panuzio, who described the county as "basically Reagan country."

Virginia Democrats have selected delegates who back President Carter by a 6-to-1 margin, but Panuzio said yesterday he thinks the president's domestic and foreign policy problems "are starting to catch up with him."