Marriott has won the competition to build a large hotel next to Washington's new convention center, Mayor Anthony A. Williams said last night. Williams is slated to make the official announcement today at the Mount Vernon Square site.

The hotel, with an estimated cost of $500 million, still must secure loans at a time when it is becoming harder to get money for large projects. The city is expected to detail at today's news conference how much it will contribute to the deal.

Marriott International Inc. and Hilton Hotels Corp. were the only companies in competition for the hotel, which convention planners believe is critical to the center's success. The center is scheduled to open in March. At 2.3 million square feet, it will be the nation's fifth-largest meeting place.

"I'm excited about it," Williams said at a campaign stop in the Deanwood area of Northeast. "The downtown is the locomotive that's pulling our city's economy, and this is adding another major engine." He said the new hotel will mean hundreds of jobs and will help to boost the revenue of the city's tourism industry

The city began the selection process in April 2001 and had expected to pick a developer by the end of last year. But the September terrorist attacks and the prolonged economic slump kept business travelers away, forcing the city to delay its decision.

Marriott officials said last night that they had not been told whether the firm had been selected. Hilton executives did not return phone calls seeking comment.

Marriott officials said last night that they had not been officially invited to today's news conference. But company spokesman Tom Marder said: "We're very pleased with this word from the mayor. We look forward to . . . a strong partnership with the District."

A hotel consultant who has worked on convention-center hotel deals but asked not to be identified because he does business with the hotel companies, said yesterday that Marriott was the "shoo-in from the beginning." Having Hilton's proposal, he said, helped city officials get leverage and a better price.

Bethesda-based Marriott, teamed with JBG Cos. and developer Kingdon Gould, originally proposed a 1,500-room hotel with 90,000 square feet of meeting space in the 900 block of Massachusetts Avenue NW, on property that Gould owns.

Hilton, of Beverly Hills, Calif., and Landmark Organization, with local developer Douglas Jemal, proposed a 1,400-room hotel with about 120,000 square feet of meeting space on Jemal's property at Seventh Street and New York Avenue NW.

"It certainly helps that they are a local business and they've contributed to the city and community," Williams said of Marriott. "The other group had local involvement as well. It clearly was the economics of the deal."

The convention center's sales staff said it already has 162 meetings planned for the new facility from 2003 through the next 12 years, bringing 1.6 million attendees who are expected to spend $1.4 billion. In its first three years, the center will host about two dozen shows each year. The shows are expected to draw larger numbers of conventioneers and attendees who spend more on hotel rooms, restaurants and shopping during their stay.

The center's sales staff said a dozen more groups, with an estimated potential spending power of $167 million, have expressed interest in coming to the District if it has a hotel next to the center.

Both developers and the hotel companies had asked the city for a subsidy because it is difficult to get financing for such a large and risky project. The city could either put up a direct subsidy for the hotel or enter into a public-private deal in which it would issue bonds through a separate financing authority to fund most of the project.