Santa Claus came early to Alexandria this week, posed on a staircase in Old Town in the glow of a marble-framed fireplace. Doorways were decorated with wreaths and garlands and artificial snow drifted delicately down over a pair of shoes poised on a doorstep.

And there was an abundance of gifts, all part of colonial Christmas glory created by advertising executives of Sears, Roebuck and Co. who chose Alexandria as the backdrop for 60 pages of its 1983 Holiday Wish Book. It will be received--and pored over--by consumers in more than 14 million households this fall.

"You just can't beat this for free advertising," Alexandria Mayor Charles E. Beatley Jr. said yesterday of the upcoming Sears catalogue. "This is going to be tremendous."

"We thought Alexandria was nice," Maynard Kessler, a Sears catalogue advertising executive, said yesterday in a telephone interview from his office in Chicago. "Shooting on location is an attempt by retailers to bring new excitement into their presentations," he said. "We were satisfied with our outing in Alexandria."

Barbara Janney, director of the Alexandria Tourist Council, said Sears agreed to identify in the catalogue the scenes photographed at historic locations in Alexandria, such as the Lee-Fendall House, an 18th-century mansion at 429 N. Washington St., and Robert E. Lee's boyhood home, at 607 Oronoco St. It seems Alexandria is not unknown to catalogue and magazine makers. Oldsmobile has been there Janney said, and the British equivalent of House & Garden is on location there now.

Janney said she and her colleagues at the tourist council, credited with much of the work that lured Sears to Alexandria, wined and dined its executives when they visited the city a month ago. It was not just the quaint colonial setting that interested the busy and harried crews, Janney said. It was also the city's restaurants, ample motel space and quick access to National Airport.

Last year Sears executives choose Cypress Gardens, a 23-acre tourist attraction in central Florida, for the background of its summer catalogue. A Cypress Gardens spokesman said yesterday that the exposure was "very positive." Bob Brody, co-owner and operator of Infinite Color Inc., a year-old custom photo lab in Alexandria that processed Sears' film, said his exposure to Sears was not only positive, but profitable as well.

In the 10 days it took the Sears crew of 34, including 21 models, to shoot about 600 rolls of film, Brody and his partner, Ihor Makara, had their own Christmas in May--a chance to make $3,000 processing the Sears work.

"I'm sorry to see them go," Brody said.