The U.S. Postal Service has asked two unions representing most of its organized workers to accept a three-year wage freeze -- a request the unions' leaders jointly dismissed yesterday as "an insult to the 500,000 people we represent."

The USPS request which was revealed yesterday, was made Friday night, less than four days before the current three-year pact is scheduled to expire at midnight Monday.

Postal Service officials confirmed yesterday that the wage offer was the first it has made thus far in early a month of acrimonious bargaining with the American Postal Workers Union and the National Association of Letter Carriers, the two largest of four unions negotiating with the government.

However, assistant postmaster general Walter E. Duka said yesterday that the wage freeze was required under provisions of the Postal Reorganization Act, which says the wages of postal workers must be based on comparability with workers in private industry. Duka said comparability has been achieved. The total cost of the USPS proposal, even with no wage increase, is $2.5 billion, he said, and would add 3 cents to the price of mailing a letter.

APWU and NALC have been seeking pay and benefit increases ranging from 41.3 percent to nearly 50 percent over the next three years, depending on a host of variables, including inflation. Union officials argued yesterday that the USPS proposal would reduce their members' real wages -- the value of the dollar based on inflation -- by as much as 17 percent over the next three years.

As expected, the Postal Service's offer brought a new round of qualified strike threats from APWU President Morris (Moe) Biller and NALC President Vincent R. Sombrotto, who summoned reporters to bargaining headquarters at the L'Enfant Plaza Hotel here to vent thier outrage. Federal law prohibits a strike by postal workers.

Postmaster General William F. Bolger "is trying to bring postal workers to their knees," Biller said. "He is making the postal workers an offer he knows they will not accept."

"This is his final and most devastating insult," Sombrotto said in his attack on the postmaster general. "What he's actually done is to roll back the clock. He's asked postal employes to give back money."

Both men urged Bolger to "get serious" if he wants to avoid a postal strike at midnight Monday. But both men also qualified their strike statements and avoided saying anything that could be interpreted as setting a definite strike deadline.

"There are no magic hours, except in fairy tales," Sombrotto said.