Mayor Marion Barry yesterday opened a new 400-bed minimum-security facility at the District's Lorton Reformatory in Fairfax County, saying that it will relieve some pressure in the city's overcrowded prison system but that still more space will be needed to accommodate an expected rise in the prison population.

The mayor only recently and reluctantly came to the conclusion that he will have to build another prison, and he emphasized yesterday that he really would prefer to work on crime prevention.

"Our D.C. jails are bulging over, but again, until we can prevent crime and get our young people and others to stop committing crimes, we are going to have to build additional facilities, unfortunately," Barry told a group of city officials and reporters.

Barry arrived by helicopter for the opening ceremonies, took a quick tour of the new buildings and had an early lunch of fried chicken and salad, prepared by the inmates at the facility's dining hall.

The new minimum-security facility, one of eight facilities now at Lorton, has eight buildings, including four dormitories and a gynmasium/culinary building. It cost $7.1 million to build and is estimated to cost $3.3 million a year to operate.

It will house prisoners in a work-release program who have less than a year of their sentences to serve. Barry said most of those at the new facility work at jobs in Fairfax County and pay part of their room and board costs.

Construction of the facility at Lorton was part of a plan for relieving overcrowding at the D.C. Jail, required under a court order. But Barry predicted that the jail will fill up again as fast as inmates are removed.

"My philosophy about the jail is that . . . as soon as we get the numbers down to 2,200 or so they'll go up rather fast. These judges see these numbers and they'll continue to fill it up. I would suspect that in another four or five months we'll get the numbers down and then they'll go back up again," the mayor said.

The new facility raises the capacity at Lorton to 3,903, while the D.C. Jail in Southeast was designed to hold a maximum 1,378, for a total of 5,281. The city's prison population yesterday was 6,154 -- 3,605 at Lorton and 2,549 at the jail, according to the D.C. Department of Corrections.

Barry said the city has added 1,887 prison beds since fiscal 1980 -- a 56 percent increase over 1979 levels.

The mayor said after the ceremony that his administration is working on a prison profile at Lorton to see what wards of the city the inmates come from and what city high schools they attended to see if any particular schools should be targeted for crime prevention programs.