The last holdout in the Nicaraguan civil war, which ended in that Central American nation 10 days ago, finally ran up the white flag on Ellicott street NW here yesterday and surrendered.

Guillermo Sevilla-Sacasa, Nicaragua's long-time ambassador to the United States and a brother-in-law of ousted Nicaraguan dictator Anastasio Somoza, capitulated and began moving his family's belongings out of his country's embassy.

Sevilla-Sacasa, the portly dean of the Washington diplomatic corps, had balked at handing over his country's elegant, but deteriorating embassy at 3200 Ellicott St. NW to representatives of the Sandinista-supported Nicaraguan provisional government.

But yesterday, while the youthful Sandinista supporters hinted broadly at a news conference that records they found in the embassy show widespread corruption involving Americans with connections to the Somoza regime, one of Sevilla-Sacasa's sons, Luis, 32, helped movers load a van that he said would take the family's belongings to an undisclosed location in Florida.

However, Ambassador Sevilla-Sacasa, a ubiquitous fixture on the embassy party circuit here for the 36 years he represented his government, was nowhere to be seen as the movers loaded such personal effects as toilet seats, clothes, plastic plants and pots and pans into the van.

One set of Sandinista supporters checked the ambassador's belongings as they were boxed, and another team perused the boxes as they were put on the van to make sure that nothing was being taken that the new government might want.

Luis Sevilla said that his father "probably has mixed emotions (about leaving) when he's been here this long."

But Michael Maggio, a lawyer representing the new government here expressed a different view at the press conference in the embassy's musty, one-time library.

"Mr. Sevilla-Sacasa," said Maggio, "had a hard time coming to grips with reality. But after a number of hours Mr. Sevilla-Sacasa retreated from all his demands."

Sevilla-Sacasa, according to Maggio, had wanted to keep the Sandinista supporters out of the embassy until yesterday, although he allowed them in Wednesday night, and also to control the locks and keys to the mansion and who came into the embassy.

"Our position was to be courteous but firm," Maggio said of the negotiations with Sevilla-Sacasa. Maggio and Sylvan Marshall, the Sevilla-Sacasa family attorney, said that the ousted ambassador and his family now have a Monday noon deadline to leave the embassy.

While Maggio said Sevilla-Sacasa is now cooperating in the transfer of the embassy's occupancy, strains of mistrust were evident everywhere at the embassy.

Luis Sevilla-Sacasa said that the new embassy occupants kept family photos "and my father's personal letters," and added, laughing at what he perceived to be the absurdity of the thought, "They think they're going to find a mint."

Maggio and the new government's charge d'affairs, Dionisio Saul Arana-Castellon, said they and the Sandinista supporters have started sifting through 30 years of embassy papers and so far have found everything from thank you notes for flowers that Sevilla-Sacasa sent to friends to documents that "indicate there may be violations of law."

Maggio, declining to make specific allegations until more of the papers have been studied, said that the possible criminal violations involve "financial matters, fraud" that benefitted the Somoza regime. "It appears there are some Americans involved," he said.

Maggio said whatever information the new embassy occupants find that might involve criminal acts will be turned over to the Nicaraguan Minister of Justice and the U.S. Justice Department. He also said that Sevilla-Sacasa has agreed to let representatives of the new government look at various items he has already shipped to Florida.

The change of personalities at the embassy was but one example of the Sandinista takeover. A portrait of the late Gen. Augusto Cesar Sandino, the father of the anti-Somoza rebel movement, hung over the embassy's fireplace and a Nicaraguan map with "Libre" splashed across it was displayed in the foyer. CAPTION: Picture, GUILLERMO SEVILLA-SACASA...moving family to Florida