Seven inmates at Lorton Reformatory's Youth Center 2 were injured Sunday night in a bloody gang fight in which prisoners swung boards, tree limbs and knives at each other. The riot, the first at the reformatory in six years, was finally quelled by prison guards, who stormed the building firing Mace and carrying shotguns.

Though details were still sketchy yesterday, a spokeswoman for Mayor Marion Barry said that the 9 p.m. disturbance at the District of Columbia facility in southern Fairfax County lasted "an hour to an hour and a half" and that about 20 inmates were involved in the fight. The fracas, apparently prompted by a long-simmering dispute over inmates stealing each other's clothes erupted in the Youth Center's gymnasium during a movie.

Prison guards and inmates said, however, that at least 80 of the 139 inmates housed in the two-story, minimum-security facility were involved in a riot that lasted four hours.

Both city and prison sources confirmed that 20 of the inmates were removed from the Youth Center after the incident and placed in control cells at Lorton's minimum-security facility across the street.

The seven injured inmates, who mostly suffered minor bruises and cuts, were taken to D.C. General Hospital for treatment. One of them, Tyrone Jones, was listed yesterday in serious condition with knife wounds.

Bobby Foster, a convicted felon at 21, said he was sitting on his gray-blanketed prison cot at about 9 p.m. Sunday night, leaning his head back against the wall, "not doing much of nothin', like usual," when "all hell broke loose.

"All over the compound, everything was noise, screaming and hollering and crying and carrying on," Foster said. "People were going after each other with knives and tree limbs and two by fours. It was like something out of Caeser's time, like a gladiator thing. There was blood on the floor. There was death in the air man, you dig?"

In the North Tower, one of four brick watchtowers looming 50 feet above the Youth Center compound, Lee Worthum's radio began "cracklin' away with calls from all over the compound about a fight." The veteran prison guard said he shouldered his Remington five-shot, semiautomatic shotgun and trained it on the yard below him.

"The whole compound erupted into a fight," Worthum said. "A long line of inmates ran into the yard. . . . I called the control center office and asked if I was supposed to fire. They said no. Several inmates and [correctional] officers ran to my tower. They were looking for safety."

Prison guards, who walked off their jobs in a 48-hour wildcat strike in September to protest staff shortages and city-ordered layoffs, were quick to attribute Sunday night's incident to manpower shortages. "It was a direct result of the shortage," said American Federation of Government Employees shop steward Ken Bynum. "We don't have enough people to get the job done. Sunday night, we only had five men inside the building and 11 outside. Usually we have 21.

"When the riot started, inmates were looking around for guards to protect them. There just weren't enough to handle it. . . . The officers were lucky that they have a good rapport with the prisoners. Otherwise, they probably would have turned on us."

Bernard Demczuk, the AFGE shop steward at the D.C. Jail who led September's wildcat walkout, said that he warned the D.C. City Council, in a meeting on Sept. 26, that "something like this would happen.

"These types of incidents will continue and will even escalate. Unless the mayor starts hiring more guards, we're going to see more outbreaks, riots and violence in the future."

Demczuk also charged that the city's negotiations with the union, which began after the strike, have been "on-again-off-again. They keep saying they're going to start hiring this fall, but it's fall now and nobody's been hired."

A spokeswoman for Barry said, "Mr. [Delbert] Jackson [D.C. Corrections Department chief] would have to make a determination as to whether a staff shortage was responsible for the disturbance" at Lorton.

The Youth Center, which was built in 1972, is one of two barracks-like dormitories that house offenders between the ages of 18 and 26. Offenders there have been sentenced under the Federal Youth Corrections Act, which provides for indeterminate sentences.

Sunday's riot was the first such incident since December 1974, when about 80 inmates in the maximum-security facility took 10 guards hostage and attempted a mass escape. The Christmas Rebellion, as it came to be called, ended peacefully after 20 hours when prison authorities agreed to negotiate with inmates over prison conditions.