Marriott Corp. said yesterday it has purchased 97 percent of the stock of rival food-service provider Saga Corp., nearly completing the $500 million deal that makes Marriott the Washington area's largest company, surpassing Martin Marietta Corp.

The purchase of Saga also makes Marriott the country's largest food-service company, according to restaurant industry analyst James M. Meyer of Janney, Montgomery, Scott.

"They've gone from being a mid-size contract food company to being the largest in the U.S. in two years," Meyer said. "It puts them in a better strategic, competitive position, because, in this business, size does matter."

In a related development, Charles A. Lynch, chairman of the board and chief executive officer of Saga, yesterday resigned effective June 30. "The merger of Saga into the Marriott Corp. . . . does not provide a meaningful role for me," Lynch said.

Marriott's quest for Saga began in early May, with an original offer of $34 a share, or about $435 million, for the Menlo Park, Calif., firm. After Saga's board and shareholders rejected that offer, Marriott raised the price to $39.50 a share.

Marriott said yesterday it would continue to purchase Saga shares at that price through Thursday.

Market analysts have said that Marriott and Saga would make a nearly perfect union, particularly in the contract food-service business. Last year, Marriott's food services to airlines and corporations represented 37 percent of the company's total sales of more than $4.2 billion.

Saga, with $1.3 billion in annual sales, is a major provider of food services to college cafeterias, corporations and hospitals. Saga also owns and operates restaurants including Stuart Anderson's Black Angus Cattle Co., The Velvet Turtle and Straw Hat Pizza.

Marriott has made it clear that its first interest in Saga is for its contract food operations, rather than for its restaurants. Analysts have said they believe that Marriott probably will sell off most, if not all, of the Saga restaurants.

Combining the existing Marriott and Saga operations will produce a $5.5-billion-a-year business. Even after disposing of Saga's restaurants, Marriott will be well ahead of $4.4 billion Martin Marietta, an aerospace company, as the biggest business in town.

The Saga purchase could dilute Marriott's earnings for 1986 and early 1987, although that may depend on what Saga assets are sold and whether Marriott can lower its overhead by combining duplicate services and management, according to analysts.

Saga will operate as a subsidiary of Marriott under the direction of Francis W. Cash, a Marriott executive vice president in charge of food service management and Host, the company's airport terminal restaurants and shops. Cash will direct the operations from the company's Bethesda headquarters, according to Marriott spokesman Robert T. Souers.

"We'll go in and take a look, see how things fit together and what steps we'll have to take," said Souers when asked whether Saga executives and employes were expected to resign or be asked to leave.

Marriott's stock, which closed at $37 a share yesterday on volume of more than 530,000 shares, split 5-for-1 effective June 20. Market analysts have attributed the strong growth in the stock since the beginning of the year to expectations of a healthy domestic tourism market this year and to good management.