Guillermo Sevilla-Sacasa, dean of Washington's diplomatic corps and for more than 20 years a virtual monument on the champagne and caviar circuit, made his last stand yesterday as Nicaragua's ambassador to the United States.

Although the keys to the Nicaraguan chancery offices were quietly handed over to representatives of Managua's new provisional government at 10:30 yesterday morning, Sevilla-Sacasa steadfastly refused to let supporters of the Sandinista-backed regime take over the embassy residence at 3200 Ellicott St. NW.

Sevilla-Sacasa, brother-in-law of deposed Nicaraguan president Anastasio Somoza, kept the new government's people waiting outside through the afternoon as he called his attorneys, called the State Department, called the White House, called ambassadors from other Central American countries.

"Do you know what an important personality I am?" he demanded repeatedly of the new government's attorney, Michael Maggio.

"The Bunker [Somoza's Managua fortress] has fallen," sighed one by-stander during the long afternoon. "This is the last place that hasn't given up to the Sandinistas."

The representatives of the new Nicaraguan government were left sitting outside in the rain until almost 6:30 p.m., when Sevilla-Sacasa finally capitulated to State Department pressure and allowed them to enter the house. But neither the former ambassador nor his family left, and Maggio said they could stay if they wanted until Friday.

"It's pitiful really," said Maggio, who was appointed to handle such matters by the new Nicaraguan charge d'affairs Dionisio Saul Arana-Castellon. "Arana's view is that we should treat Sevilla-Sacasa with the kind of humanity and respect that the Somoza dynasty denied the Nicaraguan people."

The purpose of the new government yesterday was simply to "make clear who owns the house and who is the guest" and to make sure that no Nicaraguan property or papers were removed or destroyed, Maggio added.

Neither Sevilla-Sacasa nor the numerous members of the family at the house yesterday would talk to the press, but they were visibly shaken by the turn of events.

The representatives of the new government who arrived to take over were the same people who were forcibly ejected from the Nicaraguan chancery -- at Sevilla-Sacasa's urging -- after occupying it for an hour last week, Arana, formerly a University of the District of Columbia mathematics teacher, was arrested during that incident for assaulting a U.S. officer, though the charges were dropped the next day.

Maggio said Sevilla-Sacasa expressed concern that the many women of the household would not feel safe if supporters of the new government were allowed to stay there overnight.

"I told him the Secret Service could station guards outside their rooms," Maggio said.

Many of the servants were concerned that the new government would confiscate their property. "I pay for the lock on my room, the air-conditioner, the rug, everything," one pleaded. "The [Nicaraguan] government and him [Sevilla-Sacasa] no paid nothing. "The new government's representatives assured them they would not have to worry.

Last night the new regime began the lengthy task of sorting through all official papers and Nicaraguan property in the embassy -- nothing but old letterheads had been found in the chancery.

They said they were especially in interested in finding papers relating to the Somoza family's vast business holdings and the former dictator's relationship with various members of the U.S. government.

Though Sevilla-Sacasa held out until the last possible minute yesterday, the appearance of the grounds around "Ayrlawn," as the estate is called, suggested he has been planning to leave for some time.

The grass was uncut, the swimming pool unfilled, the flagstaff without a banner. By the time the keys to Sevilla-Sacasa's long black limousine were handed over to the new government's representatives, the license plates -- which used to read DPL-1 -- had been removed. CAPTION: Picture, Michael Maggio, left, negotiates with Raul Chavez, Nicaraguan counselor, for transfer of embassy. By Douglas Chevalier -- The Washington Post