Mayor Marion Barry, under increasing pressure to relieve chronic overcrowding in the city's prisons, declared a prison emergency yesterday that will allow the District to begin the early release of about 350 inmates by the middle of next week.

Under the emergency release process, Barry is permitted to reduce by 90 days the minimum sentences of some inmates convicted of nonviolent crimes, making them eligible for parole almost immediately. Release of all but a few inmates would have to be approved by the D.C. Parole Board, under the emergency release bill that Barry signed June 22.

In a statement released yesterday, Barry said he was invoking the emergency release law because the city's prisons remain overcrowded despite "prudent management steps" taken by corrections officials. "We don't control the inflow and the outflow of criminals," Barry said. "We have done everything we can do."

District officials emphasized that inmates who were convicted of violent crimes -- including homicide, rape, assault with a dangerous weapon and armed robbery -- and those serving mandatory sentences on drug convictions will not qualify for the early parole.

Corrections Director Hallem H. Williams Jr. said he did not know the specific crimes of the inmates eligible for early parole, but said the crimes would include unauthorized use of a motor vehicle, petty larceny, simple assault and some drug offenses, such as drug possession.

No personal information was available yet on the eligible inmates, but Williams said that the average District prisoner is between the ages of 19 and 34, with less than 11 years of education, and a poor employment history. More than 70 percent of the city's prisoners have tested positive for drug use, he added.

"Nobody is going to be released who would not have been released in days, weeks or a couple of months," Williams said. "The only difference between what will happen under this emergency and what happens regularly is that the parole process is being accelerated. The Parole Board will not relax its standards."

Although the "vast majority" of inmates will come before the Parole Board, Barry can also reduce by 90 days or 10 percent, whichever is less, the sentences of inmates convicted of nonviolent crimes who are serving their maximum sentence. Those inmates, who either were denied parole or opted to serve their maximum time rather than be under parole supervision, are not required to come before the board.

During a tour of the Lorton Reformatory complex in southern Fairfax County Thursday, Williams was surrounded by groups of prisoners as he walked through an overcrowded dormitory at the Occoquan III facility.

More than 90 inmates sprawled on double bunk-beds spaced about one bed width apart, smoking cigarettes and listening to two blaring television sets, in a room with an official capacity for 35 prisoners. The dilapidated building, built in the 1920s, has no air conditioning; a few oscillating fans labored in the windows.

"We heard about that emergency law," said one inmate in blue prison garb. "When are they going to declare an emergency? When is it going to be implemented?"

"When am I going to get out?" another inmate asked.

Williams paused and turned to the inmates. "We're working it out right now," he said. "And do me a favor. If you get out of here, don't come back."

Statistics show that two out of three District prisoners will be repeat offenders, returning to the prison system after they are released. More than 98 percent of the city's current inmates have prior convictions for offenses different from the one for which they are serving time, corrections officials said.

According to D.C. government figures, 2,400 prisoners from the District are held in federal facilities across the country and 7,950 inmates are held in the city's prisons, which include the Lorton facilities and the D.C. Jail. Most of the overcrowding is at Lorton's three Occoquan prisons, which house 1,970 prisoners -- 689 more than the number permitted under an order by U.S. District Judge June L. Green.

District officials have attributed the increase in prisoners in the Occoquan units to court-ordered population limits at other Lorton facilities, drug arrests associated with the police department's Operation Clean Sweep, sentencing practices in the courts and parole revocations.

Federal law enforcement officials speculated that Barry invoked the emergency release law yesterday in an effort to meet a July 13 deadline imposed by Green for a detailed report on the number of inmates eligible for release under the emergency measure.

Green, who granted city officials a second one-month reprieve this week from her court order limiting the population at the Occoquan prisons, called the overcrowding a "crisis situation" and warned District officials that they must act immediately.

Although the city's corrections budget has tripled from $59 million to more than $180 million since 1979, and the prison capacity has been expanded by nearly 2,600 beds during that period, the inmate population has doubled in the last eight years.

Williams stressed that the emergency action "will not solve the prison overcrowding problem." He said that the city is planning to build an 800-bed correctional treatment facility by 1989 or 1990. The medium-security facility, which will be near the D.C. Jail in Southeast Washington, will offer drug treatment and counseling.

Williams said the city also plans to construct a 100-bed modular unit at Lorton's central prison to house temporarily prisoners displaced by renovations at obsolete Lorton dormitories.

The District will continue to increase its halfway house capacity, Williams added. Corrections officials have said that they found a city site that could accommodate from 350 to 500 inmates in a halfway house or minimum-security prison, but the site was not disclosed.

Barry partly blamed community opposition to prison expansion for the delay in expanding correctional facilities. John F. Herrity, chairman of the Fairfax County Board of Supervisors, has vowed to oppose vigorously Lorton's expansion, and says the plans are an attempt by the city to "turn Fairfax County into a concentration camp."

Rep. Stan Parris (R-Va.) introduced a bill in Congress Tuesday to repeal the emergency prison overcrowding measure, arguing that the city's plans for the early release of hundreds of inmates would pose a serious threat to public safety.

A number of states, including Texas, Florida and Oklahoma, have enacted similar early-release measures.