John T. (Til) Hazel Jr., the most prominent of Fairfax County developers, urged local builders last night to seize upon the idea of a billion-dollar outer beltway that would sweep through the periphery of Washington's suburbs and ease traffic on the existing Capital Beltway in Maryland and Virginia.

"We simply cannot allow this opportunity to go without leadership and to go down the drain by default," he told a dinner audience of about 200 members of the Northern Virginia Builders Association.

He warned against repeating what he called "the debacle" of plans for construction of Interstate 66, which was delayed by local opponents of the road in the 1960s and 1970s. "You cannot have required improvements terminated or impeded by selfish citizen activists who care principally for their own view of the world," he said.

Hazel's 25-minute speech did not mention that the outer beltway would probably be located near a multimillion-dollar, 542-acre commercial and residential project he has proposed on a prime tract of land near Manassas. The new road would probably enhance the value of the land considerably.

Asked after the speech about the proximity of the outer beltway and his proposed project, called William Center, Hazel responded: "I hope that the outer beltway will be somewhere near it." This drew loud laughter and cheers from the builders, and one man called out, "I hope so too!"

On another matter, Hazel said he thinks Fairfax County is poised on the brink of a "no-growth panic . . . . I think the next county board election in Fairfax in 1987 is going to be a no-growth election."

He said the backlash against development would be fueled by voters' frustration with increased traffic and by what he described as the absence of other significant problems in Northern Virginia.

Many Fairfax politicians agree with Hazel's assessment, and most agree that transporation problems and solutions will dominate the 1987 elections, when all nine members of the Fairfax County Board of Supervisors will be up for reelection.

In an interview after the speech, Hazel tied a Fairfax Planning Commission recommendation yesterday that would limit one of his his largest projects to what he called "a frenzy" of antidevelopment sentiment.

The project, called Virginia Center, calls for nearly 2 million square feet of office and retail space on a 61-acre tract near the Vienna Metrorail station, scheduled to open June 7.

The commission's recommendation would cut Hazel's proposal roughly in half although he would still be permitted to build several 19-story office towers on the land.