The Alexandria City Council gave the McDonald's hamburger chain a break yesterday and voted to permit the giant corporation to open a restaurant in Old Town.

The council voted 5-2 to permit the fast-food chain to open a 150-seat McDonald's on King Street, near the busy Washington Street intersection. The building is now occupied by Pier I Imports.

"The restaurant will be tastefully decorated. In fact, it will be in the pseudo-colonial federal style that is inkeeping with much of Alexandria," McDonald's attorney Bernard M. Fagelson laughingly told the council.

In contrast to last week's meeting of the City Planning Commission, where a dozen Old Town residents spoke against the restaurant proposal, there was virtually no opposition expressed at the council meeting yesterday.

Council members Donald C. Casey, a Democrat, and Marlee Inman, a Republican, cast without comment the only votes against the proposal.

McDonald's formally agreed not only to limit its restaurant to one floor and a mezzanine,, but also to close no later than 10 p.m. each night and to send employes out on a regular basis to pick up the distinctive McDonald's wrappers and other debris discarded on either side of King Street near the restaurant.

The famous Golden Arches symbol of the chain, which has sold more than 30 billion hamburgers since it was founded in 1955, will be banned from Old Town. But the company will be permitted to display its name in 21-inch high white letters on the outside of the beige brick building.

Opponets had said before the meeting they feared the restaurant featuring low-price food would mar the traditional character of the Old Town area by attracting excessive foot traffic and teen-agers who might loiter in frount of the area's expensive shops and boutiques.

Fagelson argued that the more than 7,000 employes in offices near the intersection would be the primary users of the restaurant.

"It isn't going to be a teen-age hangout, but a typical urban type of place," Fagelson said. "We're not suggesting people will get gourmet food, but they will get good food at a reasonable price."

Fagelson estimated that four out of five McDonald's customers would dine inside the 13,000-square-foot restaurant, rather than carry their orders out.

Attorney Yvonne Weight, who earlier had argued that "Big Macs and special sauce are not compatible with the old and historic area" of Alexandria, yesterday simply asked the council to check the site every six months to make sure the restaurant picks up its trash.

Fagelson said the restaurant could open for business at the beginning of summer, 1980.