D.C. Mayor Marion Barry was admitted to the coronary unit of Howard University Hospital early yesterday after complaining of chest pains and a shortness of breath and is expected to stay there at least a week to undergo tests.

He was rushed by ambulance to the hospital from the nearby Washington Hilton Hotel, where he had been attending a weekend political gathering.

Hospital officials said late yesterday that the 47-year-old mayor was in "guarded condition," resting comfortably under sedation and undergoing extensive tests for everything from severe digestive problems to a possible heart attack.

"We currently have no positive tests that will confirm any diagnosis," said Vincent Roux, the hospital's medical director. "Because of the seriousness of the situation, the mayor was admitted to the coronary unit for observation. . . . We certainly don't expect the mayor to be carrying out any duties for the next week."

Roux said Barry has no medical history of problems with his heart, arteries or digestive system and doesn't have high blood pressure. Without ruling out the possibility of a heart attack, Roux said Barry's problems could have been caused by a stomach disorder.

"Many things cause chest pains," he said.

Aides to the mayor later insisted that Barry's hospitalization would do little to disrupt the day-to-day operation of the District government. "The situation is that the mayor is alert and not in any way incapacitated," said Annette Samuels, the mayor's press secretary. "He will be able to make decisions as he has in the past."

One aide, who declined to be identified, said Barry planned to spend a little time yesterday afternoon tending to city business from his hospital bed.

"The mayor is in charge," Clifton Smith, the mayor's staff director, declared as he left the hospital yesterday.

City Council Chairman David A. Clarke, who was among a handful of city officials who were permitted to visit briefly with Barry and the mayor's wife, Effi, at the hospital, said Barry appeared alert and able to speak.

"He certainly knew who I was and was able to joke," Clarke said. "He said maybe we City Council members have been working him too hard . . . He looked exhausted."

Samuels said that when she visited Barry's private room, "he was talking to Mrs. Barry and sipping orange juice."

Barry, who usually maintains a heavy work schedule and who has had a weight problem in recent years, stayed over at the Washington Hilton on Friday and Saturday nights to participate in the 13th Annual Congressional Black Caucus legislative weekend.

The mayor officiated at the opening of a black caucus tennis tournament Saturday morning, at the Shoreham Hotel, but didn't stay around to compete.

That evening, Barry and his wife took part in a series of elaborate cocktail receptions and an awards dinner at the Washington Hilton, at 1919 Connecticut Ave. NW.

Barry delivered the opening remarks at the dinner and spent part of the evening in the company of Deputy Mayor Ivanhoe Donaldson, the mayor's top political adviser, former city administrator Elijah B. Rogers and former Department of Human Services director James Buford.

According to one observer at the dinner, the mayor seemed uncharacteristically subdued as he made the rounds of the tables, even while he chatted with former vice president Walter F. Mondale, a leading candidate for the Democratic presidential nomination.

After the dinner, Barry hosted a private reception in his hotel room that went on well past 3 a.m. yesterday, according to an aide to the mayor. Mrs. Barry, who stayed with her husband at the hotel Friday night, left the hotel Saturday night to return to the Barrys' Suitland Road SE home, according to Samuels.

The mayor began experiencing chest pains and breathing problems around 8 a.m. yesterday, shortly after he arose to get ready for a morning meeting, according to Samuels.

The D.C. Fire Department immediately dispatched an ambulance and a special truck equipped with advanced life-support equipment to the hotel after receiving the mayor's call for assistance. A police officer said he saw Barry standing outside the main entrance to the hotel before the ambulance units arrived. Barry looked extremely tired, the officer said, and needed help getting into the ambulance.

The ambulance arrived at the hospital, located nearby at 2041 Georgia Ave. NW, at 8:16 a.m., according to hospital authorities. Barry was wheeled into the emergency room on a stretcher.

"The mayor came into the emergency room complaining of shortness of breath and mild to moderate chest pains," said Roux, the medical director.

Roux said the chest pains became increasingly severe and then subsided. "When I talked to him an hour after he arrived, he didn't really have any specific complaints," he added. "The chest pains were no longer present."

Barry will remain in the coronary unit for 48 hours while doctors attempt to diagnose his problem and is receiving oxygen through a tube in his nose. Dr. Charles Curry, chief of cardiology, is the mayor's attending physician.

Barry was last hospitalized in serious condition in March 1977, after he was shot in the chest outside the City Council chambers during the takeover of the District Building by Hanafi Muslim terrorists. Barry, who at the time was a member of the City Council, survived when the small-caliber bullet just missed his heart.

Under the city charter and provisions of a new government reorganization plan that Barry implemented last January, City Administrator Thomas Downs--one of three deputy mayors--would be in charge of running the city in the mayor's absence.

Barry, a former civil rights activist who rose to political power during the 1970s, was elected last November to a second four-year term as mayor.

The mayor is empowered by the charter to delegate his authority to a subordinate during periods of disability or absence from the District, except for signing legislation and city contracts with the federal government.

By law, the chairman of the city council would automatically succeed a mayor in the event of his or her death or removal from office, until another election could be held.