A gleaming white model of the Washington Monument towered over a lavish spread of hors d'oeuvres last night as more than 400 business executives, politicians and potential conventioneers gathered at Washington's new $98.7 million Convention Center to toast its grand opening.

The guests, including 100 out-of-town trade association executives who were being wined, dined and wooed by convention center officials, were still buzzing about Wednesday's siege on the Monument grounds that ended in the death of a 66-year-old nuclear arms protester.

"We watched that on TV Wednesday evening hoping it would be over by the time we got here," said Lorraine Cullison of Atlanta, wife of an official of the Technical Association of Pulp and Paper Industries.

"I think the police did what they had to," said James E. Brophy, executive director of the American Association of Orthodontists.

Brophy and the other visiting trade association executives are on a three-day junket paid for by the convention center, several major airlines and Washington hotels and limousine services. They have been whisked around to receptions, VIP tours of the Decatur House and the Capitol and, last night, to the black-tie reception and dinner at the sprawling new convention center, located on New York Avenue between 9th and 11th streets NW.

Many of the guests said they were impressed with the size and flexibility of the 800,000-square-foot center and would urge their groups to meet there.

"They really got their act together," said Oscar S. Glasberg, a New York executive in the glass and metal industry. "Before now, Washington wasn't a convention city. I remember visiting the sight about a year ago. It looked like hell. Now it looks great."

George W. Demarest Jr., the congenial general manager of the center, patted backs and gently squeezed arms as he worked the crowd of prospective clients, who were treated to steak tartare, fried octopus, shrimp and fresh strawberries.

"The people in this room represent the next 10 to 15 years of business," Demarest said. "These are people we need to impress. I think it's working very nicely."

Michael C. Rogers, the deputy center manager, added, "We're using a very soft sell--but one with a touch of elegance befitting Washington."

Mayor Marion Barry missed the reception, but was expected to address the gathering later in the evening, after a sit-down dinner of filet of beef and truffles and four different wines.

Earlier yesterday, while the splashy Cardozo High School marching band offered a drum roll and several hundred guests shivered against the cold, Barry and C&P executive Edward Singletary snipped a ceremonial ribbon outside the convention center.

The exuberant Singletary, chairman of the convention center board, mangled the lyrics to a Rodgers and Hammerstein tune from "Carousel" to make the point that, "Even though it doesn't feel like June, the convention center is spreading out all over."

And Barry pronounced the event "an historic occasion in the District of Columbia" and boasted that the center was completed on time and within budget.

But the handful of construction workers peering down from atop the building, near New York Avenue and Ninth Street NW, were a testament to the fact that plenty of work remains to be done before the first convention-goers pass through the center doors in early January.

The ribbon-cutting outside the Ninth Street entrance to the center drew nine members of the D.C. City Council, D.C. Del. Walter E. Fauntroy and scores of local business executives, civic leaders and labor officials, many of whom are convinced the long-awaited project will provide a major boost to the city's economy and hotel industry.

About 30 members of the ribbon-cutting committee crowded onto a large red-carpeted podium to pose for a group photograph.

But after the ribbon was snipped and Barry and Singletary offered brief remarks, the crowd hurried inside to ward off the cold and get their first look at the cavernous new center. Inside, Barry and Singletary mounted another podium to resume their oratory, but many of the guests drifted to the other side of the center to take advantage of the rather skimpy array of hors d'oeuvres but generous supply of liquor.