On the same day that President Reagan and hundreds of others paid homage to George Washington at his Mount Vernon tomb, a small group of people gathered to officially memorialize the previously overlooked graves of Washington's slaves.

Once overgrown by vegetation, the burial site for an estimated 300 of Washington's slaves was refurbished recently after remaining in obscurity for more than 50 years. The Mount Vernon Ladies' Association, which oversees the Washington estate, cleared the area, installed two park benches, laid a gravel path and opened the burial site to tourists this week after the association was criticized for neglecting the site.

"We welcome the opportunity to work with the Ladies' Association," Jerry Langley, president of the Fairfax chapter of the NAACP, said during yesterday's ceremony. " . . . What they've done is a good beginning but there's a lot more to be done."Attending yesterday's ceremony were about 30 people, including Fairfax County Board of Supervisors member Sandra Duckworth, who represents the Mount Vernon area, and Board Chairman John F. Herrity.

The observance was the first official recognition of the site since 1929 when the Mount Vernon Ladies' Association placed a rectangular stone marker that read: "In memory of the many faithful colored servants of the Washington family buried at Mount Vernon from 1760-1860. Their unidentified graves surround this spot."

During the brief ceremony, Judith Burton, a Fairfax school teacher, laid a wreath of flowers at the slave burial ground, which is about 50 yards southwest of Washington's tomb on a wooded knoll overlooking the Potomac. Burton said later that she believes one of her ancestors, West Ford, worked at Mount Vernon after Bushrod Washington inherited the estate following Martha Washington's death in 1802.

John A. Castellani, director of the Ladies' Association, also announced yesterday that his group had begun a campaign to raise $350,000 to conduct a historical and archeological study of the burial site.

"The 316 citizens buried in that ground probably contributed as much to this nation as the man whose birthday we celebrate today," said William E. Carr, a psychologist and local historian who attended the ceremony.

The effort to improve public awareness of the site follows recent publicity about its obscurity and poorly maintained condition. Frank Matthews, a representative of the Fairfax NAACP unit, also raised the issue during a county board hearing on a Mount Vernon Inn property tax exemption the association is seeking from the state and county.