The federal government capitulated yesterday and agreed to keep a decrepit downtown Washington shelter for the homeless open indefinitely past tomorrow's planned closing because it has not been able to find other shelter sites.

In unusually strong language, Deputy Under Secretary of Health and Human Services Dixon Arnett chastised D.C. Mayor Marion Barry for hindering the search for new shelters through a lack of cooperation with federal officials.

"Mayor Barry and the District government should be our staunchest allies in this compassionate mission," Arnett said. "Instead, the mayor is our adversary. Unlike his city counterparts from coast to coast, he has abdicated the capital city's obligation to the homeless."

Arnett said it was "absolutely grotesque" that Barry joined Mitch Snyder, leader of the Community for Creative Non-Violence, the private nonprofit group that operates the 800-bed shelter at 425 Second St. NW, at a press conference last week to claim that it is the federal government's responsibility alone to renovate the facility.

President Reagan, prompted by Snyder's 51-day fast last fall, promised two days before his reelection to turn the facility into a "model physical shelter," a pledge that Barry and Snyder both have adamantly said the federal government should live up to. Snyder's negotiations with federal officials over the extent of the renovations broke down in mutual recriminations and the government decided to close the shelter.

Barry was out of town yesterday, but City Administrator Thomas M. Downs said, "It's important to note that the mayor is not an adversary on either side of this dispute" between CCNV and the federal government.

"The mayor's commitments to the homeless are in the budget," Downs said. The city now spends $7.3 million to operate 844 beds for the homeless, he said, and is increasing the totals to $8.2 million and 1,144 beds in the fiscal year starting Oct. 1.

Arnett said Barry "slammed the door" on efforts by the D.C. Coalition for the Homeless, a private advocacy group for the District's homeless, to use two city-owned facilities as shelters.

Arnett identified the two properties as the vacant Nichols Avenue School in Southeast Washington and a former funeral home on Eighth Street NW.

Downs said the city school system, not the District government, controls the Nichols Avenue school and that he had never heard of the funeral home property.

School board member R. Calvin Lockridge, who represents Ward 8, where Nichols is located, said no one asked school officials about using it as a shelter and that the building may be turned into a drug rehabilitation center.

Arnett said federal officials, while keeping the Second Street shelter open, will now try to find other buildings on federal property in the District to use as quarters for the 600 homeless men and women who nightly sleep in squalor at the federally owned building.

Arnett set no date for finding the new facilities, but said he hopes that it can be done "inside of a month."

He said officials were not able to contract for any new space to replace the Second Street shelter in the 10 days since U.S. District Judge Charles R. Richey ruled that the government could close the vermin-infested building, but only if it found other shelters and devised a long-range plan "to eliminate homelessness in the nation's capital."

Arnett said that since then Snyder and the CCNV "have been more concerned about maintaining their 'turf' at the existing shelter -- which Judge Richey found to be an abomination -- than in helping to find suitable alternative accommodations for the District's homeless."

Arnett said the federal government eventually hopes to locate seven to 10 relatively small shelters, one of which may be at a building owned by the Gospel Mission at 477 H St. NW, a structure that needs renovation but could eventually shelter about 100 women.

He said that even finding shelter space on federal property has been difficult. "Some in the federal government are surprised at the possibility that they'd be neighbors to the homeless," he said. "It takes explanation, time and consent."

Snyder said, "It's not surprising they've thrown in the towel [on trying to find space in order to close the Second Street facility by tomorrow]. They had no choice. If no one had pressed them, they would have closed the building and the people would have gone out to the garbage barrels in the alleys again and the feds would have wrung their hands."

Despite Arnett's announcement that the downtown shelter would remain open, lawyers for CCNV pressed for an injunction yesterday in the U.S. Court of Appeals to block the government from closing the building while they appeal Richey's decision.

Peter J. Nickles, an attorney representing CCNV, told a three-judge panel that despite the government's pledge to keep the shelter open, "I would be much more comfortable if there were an order from this court."

He also asked Chief Judge Spottswood W. Robinson III and Judges Robert H. Bork and Patricia M. Wald to order the government to make rudimentary repairs at the shelter, which he estimated would cost $200,000.

Assistant U.S. Attorney Edith S. Marshall said the government would comply with Richey's order not to close the shelter until other sites were available.

However, she opposed spending money for repairs, saying that it "would deplete the amount of funds available to find alternative shelters."

The government has promised to spend $2.7 million on the relocation effort, money it originally said would be sufficient to carry out Reagan's pledge to turn the World War II-vintage structure into a model shelter.

"Shouldn't [the homeless] at least be put in a place that isn't so dangerous?" Wald asked Marshall.

"This isn't a place where people are committed," Marshall said. "They can choose to leave that facility."

The appellate panel may issue a ruling today.