DETROIT, SEPT. 14 -- The United Auto Workers tonight extended its contract with General Motors Corp. and prepared to bargain through the weekend in an effort to hammer out a job security program it hopes will serve as a pattern for union workers throughout the industry.

Shortly before the current contract's midnight expiration date, UAW President Owen Bieber issued a brief statement saying talks were continuing and advising workers to report for their regular shift until further notice.

Both sides said they were optimistic an agreement could be reached covering some 300,000 union workers at the nation's largest automaker, with final details of a program to guarantee jobs all that was left to be resolved.

"Meaningful progress has been made in the current contract negotiations," Alfred S. Warren, a GM vice president and chief negotiator, said in a statement tonight. The two sides broke off talks shortly after midnight but agreed to resume them Saturday.

Negotiations at Ford Motor Co. and Chrysler Corp. have been put on hold while the union attempts to negotiate a pattern-setting contract at GM. Once agreement is reached there, the union will take the pattern-setting contract to the other two automakers.

If the two sides fail to reach agreement, the union is expected to order selected strikes at key parts facilities that would cripple GM production without forcing the union to pay millions of dollars in strike benefits.

The key to any settlement is the job security program for a union that has been decimated by job losses in the auto industry. In the last decade, the number of UAW jobs at GM, Ford and Chrysler has dropped by more than a quarter of a million, a number almost equal to the UAW's entire current work force at GM.

During that same period, Honda Motor Co., Toyota Motor Corp. and Nissan Motor Co. all have built lower cost, nonunion manufacturing facilities in the United States and continue to gobble up an increasing share of the domestic car market. These so-called transplants have become the competitive gauge for judging labor costs in the U.S. auto industry.

The UAW goal has been to guarantee the jobs of the 280,000 union members still working at GM and provide some hope of future employment for the approximately 30,000 union members on indefinite layoff at the company. In addition, the union has insisted on some protection against massive job erosion in the future.

GM, meanwhile, insists that it must eliminate 20,000 jobs a year over the life of the new contract if it hopes to be competitive with both the Japanese transplants and nonunion parts manufacturers at home and abroad.

In recent weeks, the two sides have been exploring an agreement the UAW reached last spring at the J.I. Case Co., an agricultural implement manufacturer in Wisconsin.

That agreement provides what amounts to a two-tier job security program and a guaranteed minimum level of employment that is determined on a plant-by-plant basis.

GM was picked as the bargaining target this year because the company makes more of its own parts than Ford or Chrysler. By picking GM, the UAW hopes to work out a job security program that will curb efforts to contract out manufacturing work.