A coalition of local and national concumer groups formally launched a counterattack yesterday against Giant Food's decision to stop marking prices on individual grocery items, announcing plans to demonstrate outside the Chain's stores and calling for a boycott.

The coalition said it did favor the new lower prices that Giant established when it began marking prices on shelves rather than items. However, Judy Kory, a representative of the Virginia Citizens Consumer Council, said that some other stores have been able to offer lower prices while retaining price markings on most items.

For example, Kory said, researchers in a spot check found that prices in a Grand Union Basics store were lower than those advertised by Giant on 24 of 62 items checked on April 9. The prices were the same on 37 items, she said, and in one case Basics was higher than Giant.

Ellen Haas, a coalition representative, said that the survey showed "item price removal was not necessary to lower food prices" and charged that Giant "has moved to deliberately deny consumers information which is critical to intelligent food shopping."

In addition to the demonstration and boycott plans, the coalition said it will press for legiation that would mandate item pricing in supermakets in Washington-area jurisdictions. Ten states, none in the Washington area, now require item pricing, according to Jim Boyle, an attorney with the Consumer Federation of America (CFA). Efforts to pass such a law in Maryland earlier this year failed.

The coalition is an umbrella alliance for 12 consumer groups, including the Maryland Citizens Consumer Council, the Virginia Citizens Consumer Council, the District of Columbia chapter of the Americans for Democratic Action, the Community Nutrition Institute and the American Association of Retired Persons.

Giant began placing prices on shelves rather than on individual items one week ago. The new policy was sandwiched into a promotion that promised shoppers lower prices on 1,500 to 2,000 items in the 126 supermarkets that Giant operates in the Washington area.

That move triggered a price war as Safeway, Giant's biggest competitor, reduced prices on 2,000 items but pledged that it would continue to mark prices on most individual items in its 125 stores. Some Safeway stores equipped with automatic scanner devices or with electronic cash registers have stopped marking prices on some items, according to Ernest G. Moore, a Safeway representative.

Fewer than half of the Safeway supermarkets in the Washington area now have the electronic check-out machinery necessary to eliminate item pricing completely. But all of the Giant stores have such equipment.

Yesterday, after the first week of lower prices and no item pricing, Terry Gans, director of corporate operations for Giant stores, said that sales were running ahead of projections. "We are seeing a gain over what we expected," he said.

But at Safeway, Spokesman Moore was making the same claim: "Our sales are up."

Moore said it is too early to tell for sure which chain is gaining in the battle for bigger market share. "Maybe we will know in another week."