Partisans of Reston were in the majority yesterday at a conference to gauge the success of the first 20 years of America's best-known planned community, but residents and outsiders alike warned that the community's original social goals are in jeopardy.

At issue in the first session of the three-day meeting was whether Reston has succeeded in attracting a diverse population, and whether, as planners in the 1960s hoped, it offers a more rewarding life style than a traditional suburb.

Reston "has provided an alternative to the urban sprawl," said Glenn W. Saunders Jr., who was chief planner for Robert E. Simon Jr., the developer of Reston.

Saunders said Reston had met its ambitious social goals of providing a broad range of leisure activities, housing and employment in an attractive setting, while also turning a profit. "We knew where we were going," Saunders said, "and most of the time we took the right road there."

The upbeat mood was broken, however, when a Reston high school student, covering the conference for his school newspaper, rose to repeat an old complaint in the community: that there is nothing for adolescents to do. The remark appeared to make some of the conferees uncomfortable.

"We talked about how to help teens have a good time and a productive time," said Simon. "But I don't know where this has been solved. I think it's a legitimate criticism of U.S.A. 1985."

In a session examining racial and economic diversity in Reston, the criticism sharpened.

Sylvia F. Fava, a sociology professor at Brooklyn College, said Reston's diversity is "inherently limited" by housing costs and "the larger realities of Amercian life." She warned that changing attitudes among the young mean that "it will be harder to recruit the next generation of Restonians committed to the goal" of racial and economic diversity.

One theme of the conference, expressed by Fava and others, was that, unlike the original "pioneers" who moved to Reston and worked for its ideals, the newer families moving there are unaware of Reston's special goals.

Most newcomers, the conferees agreed, were simply looking for a nice home in a school district with a good reputation. A number of speakers worried that this trend threatens to erode Reston's commitment to a concept some have called utopian. Some wondered if emphasis should be given to transmitting Reston's ideals through "oral history."

Fava and Kathy Lamkin, a teacher at Lake Anne Elementary School, both said that Reston downplays its black community in marketing efforts such as television advertisements.

Lamkin, who said hers was one of the first black families to move to Reston, said the community's television spots present "an alarming homogeneity of skin color and economic level." She also said that condominium conversions are shrinking the rental housing market and that federal policies are squeezing those in need of low- and moderate-priced housing in the area.

About 10 percent of Reston's residents are black, according to county statistics, while Fairfax County as a whole is about 6 percent black. Reston also has a lower average household income than Fairfax and a slightly higher percentage of families living below the poverty line.

Donald Grant, a lawyer for the Department of Housing and Urban Development who lives in Reston, said: "Middle-class blacks who move to Reston can move within the society. But there is a real question of whether low- and moderate-income people can."

"There are no . . . poor people here to speak about their concerns," Grant told the audience of about 75. "I'm always under apprehension when middle-class people . . . speak for other people."

Said Lamkin: "I'm concerned that we're becoming the land of the yuppie -- and proud of it."