The new Democratic-controlled and slow-growth oriented Fairfax County Board of Supervisors was sworn into office yesterday in a festive ceremony before 1,000 cheering guests and a sea of lights and television cameras.

Fairfax County Circuit Court Chief Judge Barnard F. Jennings administered oaths of office to the new chairman of the board, Democrat Audrey Moore, eight supervisors, the county sheriff and the commonwealth's attorney. It was one of the last official acts of Jennings, who retires tomorrow as chief judge after 23 years on the bench.

Hundreds of well-wishers, including current and former politicians, civic activists, campaign workers and county employes, crowded into the cafeteria of Robinson Secondary School near George Mason University for the inaugurations. The half-hour ceremony, complete with police honor guard, began with music by a brass quintet and ended with a buffet.

Although the swearing-in ceremony was yesterday, the new board does not officially take office until friday.

The ceremony marked the return of a Democratic Party majority to the nine-member board, which after a special election in the Mount Vernon District three years ago became the board's minority party for the first time since Reconstruction. The new board has a 7-to-2 Democratic majority; the old board had five Republicans and four Democrats.

More significant than party affiliation, however, is the new board's stance on development.

Three Republican supervisors, including Chairman John F. Herrity, lost their November reelection bids to candidates who favor slowing the county's growth.

After taking her oath, Moore smiled and waved as many in the crowd cheered and applauded.

Jennings then gave the oath to the other board members: returning Supervisors Joseph Alexander (D-Lee), Martha V. Pennino (D-Centreville), Kate Hanley (D-Providence), Thomas M. Davis III (R-Mason), Elaine N. McConnell (R-Springfield) and board newcomers Sharon Bulova (D-Annandale), Lilla Richards (D-Dranesville) and Gerald Hyland (D-Mount Vernon).

Jennings then administered oaths to Commonwealth's Attorney Robert F. Horan Jr., a Democrat, and county Sheriff M. Wayne Huggins, a Republican.

In a 10-minute speech after her inauguration, Moore reiterated the themes she has emphasized in recent meetings with area business and development leaders, warning that transportation improvements will be costly and will take a long time to implement.

Saying that an adequate transportation system in Northern Virginia could cost as much as $4 billion, Moore called for "a new era of self-reliance," adding that funding for road improvements "unfortunately and inevitably rests with ourselves."

"Our county is showing signs of stress," Moore said in her speech. "Our roads are clogged with traffic, our schools are overcrowded and development is out of balance. Many of these problems are the symptoms of decisions that have been deferred in the past. This new board will not have the luxury of ignoring the future. We will be required to address more pressing problems than faced by any of our predecessors. And we will have to address those problems right away."

Moore was interrupted by applause once, when she said school funds should not be cut "at the expense of overcrowded and unsatisfactory conditions for learning."

She said that at the board's first meeting on Jan. 11, she will propose creation of a transportation advisory committee to study what transportation improvements are needed, their costs and how the funds should be raised. Moore said after her address that she will propose that the commission consist of up to 10 local business and civic leaders, and that they report with specific recommendations to the board by May or June.

Asked if the committee might duplicate the work of the County Goals Advisory Commission, scheduled to present its final report to the Board on Jan. 11, Moore said she hopes a transportation committee would focus on ways to pay for transportation improvements. Transportation is also a primary concern of the goals commission.

Moore said the appointment of a transportation committee would not delay her request for a $150 million road bond referendum in the spring.

Also on Jan. 11, Moore said, she hopes the state Department of Transportation will recommend steps to speed road construction projects that are paid for but not under way.

She said she plans at the same meeting to ask the county executive to recommend ways the county can "streamline and speed up our own transportation processes."