District Mayor Marion Barry got a big laugh last month when he observed that his plan for a new prison would draw no opposition from the folks in the Congressional Cemetery, which adjoins the proposed site.

However, in a town that has a lobbying group for just about everyone, the mayor clearly underestimated the dead. An official of the Congressional Cemetery Association has written a letter, on behalf of the approximately 66,000 persons interred at the Congressional Cemetery in Southeast, protesting the proposed D.C. prison location as "ill advised."

The letter, addressed to Assistant U.S. Attorney Royce C. Lamberth, with copies to Barry and others, notes that the cemetery, founded in 1807, has historical significance and should not be subjected to the increased traffic, noise and "oaths and obscenities from inmates, [which are] fundamentally incompatible" with the cemetery.

"It's true the dead can't speak for themselves," commented association board member Christopher Herman. "So other people have to speak for them."

Among those whose graves would be in the shadow of Barry's proposed 700- to 800-bed prison are such worthies as J. Edgar Hoover, John Philip Sousa, Civil War photographer Mathew B. Brady and about 10 D.C. mayors from the 19th century.

"Barry would be interested in that," observed Herman.

The mayor, unveiling the proposed site at a news conference March 21, indicated on a large chart that the parcel did not directly adjoin any residential neighborhood.

"We shouldn't have any opposition over here," he said, gesturing first toward the graveyard on the south and then indicating the Anacostia River on the east. "And on this side of the facility is the river, and we shouldn't have any opposition from the river."

City Administrator Thomas Downs said yesterday the city would spruce up the perimeter of the proposed prison to make it more acceptable to the cemetery group.

"We think we can offer them a betterment over their current situation," he said. The cemetery, which Herman's group operates under a lease agreement with its owner, Christ Church Capitol Hill, adjoins the D.C. General Hospital motor pool facilities.

In other prison-related business, Barry met yesterday for more than two hours with members of the D.C. City Council to discuss differences on the prison issue.

Council members voiced displeasure at the mayor's failure to involve them in his negotiations on the prison, according to some in the meeting. Hal Williams, a staff aide who handles the prison issue for the mayor, said after the meeting that Barry told the members he would send them a prison plan "for discussion" once it is formulated, but that no timetable had been set.