Manufacturers continued to shut down operations and lay off workers yesterday, including the Mack Truck Co. which expects to idle 800 workers at its Hagerstown plant by the end of the month.

The Hagerstown layoffs are part of a nationwide layoff of 2,683 workers by Mack Truck, which operates four plants nationwide. The furloughs are the largest layoff's in the company's history.

Nearly 600 of approximately 4,000 workers in the Hagerstown plant, which manufactures engines, transmissions and rear axles for Mack trucks, have already been furloughed, with another 200 expected to be laid off by the end of May. In all, the company expects that 10 to 15 percent of its production and management employees will have been idled by the month's end.

Mack President Alfred W. Pelletier cited declining truck sales and orders as a cause of the layoffs. The company has been buffeted by economic pressures including high interest rates and a slowdown in the construction industry. Pelletier said some of the layoffs may last into 1981.

The Ford Motor Co. announced yesterday that it will suspend passenger car production in assembly plants in Atlanta, Ga., and St. Louis, Mo., next week. The company also said it will suspend light truck operations for two weeks beginning next week at assembly plants in Norfolk and Wayne, Mich., and near Minneapolis.

Ford also announced that it would keep previously shut plants in Louisville, Ky., and San Jose, Calif., closed for another week, raising the total of workers who are temporarily idle to more than 12,000.

General Motor Corp. announced it will temporarily close its Arlington, Texas, and Doraville, Ga., car assembly plants next week, idling 8,400 workers. GM will also close a Pontiac, Mich., truck plant because of a parts shortage. The Pontiac closing affects 2,700 workers.

In a related development, the Labor Department announcement yesterday that more than 49,000 workers at 51 General Motors Corp. parts plants in nine states have been certified eligible to apply for special unemployment assistance, training and relocation. The Labor Department found the workers were eligible for "trade adjustment assistance" because their unemployment resulted from increased imports.

The Labor Department denied petitions on behalf of another 3,065 workers at six other parts plants and at the Fisher Body General Offices in Warren, Mich.

International Harvester officials announced yesterday that they are laying off approximately 270 workers indefinitely beginning today. The workers are employed in the company's Canton. Ill., plant and at other plants in Iowa and Illinois.

Officials blamed the action on lost sales during a recent auto workers strike and on seasonal adjustments.