D.C. Mayor Marion Barry took Fairfax County Supervisor Sandra L. Duckworth on a tour yesterday of the city-run Lorton prison complex in southern Fairfax in an attempt to assure the suburban lawmaker that prison security had been reinforced.

Duckworth, a Democrat whose Mount Vernon District includes Lorton, has complained bitterly about security at Lorton, which holds District criminals, following a number of escapes last year.

"I feel much better now than I did," Duckworth said after the tour, from which the press was barred. "They have done a great deal of work to increase security in the last six months. I think they are making a good-faith effort to improve security at Lorton.

"It makes me feel better about the fact that they are interested in increasing security. It will make them a better neighbor," she said, adding, "But in the long run, I don't want them there at all."

Prison officials have raised the authorized guard strength from 279 to 331. There are now 311 on staff and the prison is in the process of filling the additional 20 positions.

Among the security improvements shown on the tour were new metal detectors, additional guard huts, increased television monitoring, perimeter guards in cars, 30 new walkie-talkies and double fences with razor-sharp tops.

The hands of all entering visitors will be stamped with a number, visible only under infrared light, and they will have to show that stamp when they leave. Officials hope this will prevent prisoners from changing clothes and walking out with visitors.

County figures show that since 1980, there have been 20 escapes from Lorton. Another 29 prisoners did not return from work-release programs.

The mayor also promised to establish a liaison to work with county residents living near Lorton.

Rep. Stan Parris (R-Va.) and Fairfax Board of Supervisors Chairman John F. Herrity (R) have asked city officials to move the prison to the District, a theme echoed by most county politicians. Duckworth and other county lawmakers are also opposed to the court-ordered plan to increase by 150 the number of prisoners Lorton can handle.

Barry at first told the Associated Press yesterday that he would go along with moving the prison if the federal government would pay the full cost. "We'd love to have our employes closer," he said. When pressed later, Barry refused to clarify his position, saying, "It'll never pass," and "What plan?"