The Washington Post yesterday notified 1,350 employes represented by the Newspaper Guild that it has unilaterally implemented economic provisions ot its last contract offer, which had been rejected by the union.

The Post said news and commercial workers will get immediate raises averaging 6.6 percent and will receive wage and fringe benefit increases averaging 6 percent next year and 5.8 percent in 1985.

The decision to impose the economic terms of the contract offer came after union members rejected it earlier this month and the newspaper declared an impasse in negotiations with the union. The Post issued a statement yesterday saying the newspaper "reluctantly concluded that the 14-month negotiations with the guild have reached a deadlock, and we wanted the employes in the unit to receive long overdue raises."

Martha Hamilton, chairperson of the union bargaining committee, said, "What The Post did is arrogant, abusive and shortsighted. We think it's a serious mistake that threatens serious damage to the Post's most intangible and most important asset--the way its employes feel about the place where they work."

The union is preparing to file an unfair-labor-practices complaint against the newspaper for refusing to continue contract talks, she said. "The only issue on which the two parties are far apart is on whether or not to bargain," Hamilton said.

The Newspaper Guild's contract with The Post expired in July 1982. Though negotiations produced tentative agreements on many provisions of a new contract, there was no consensus on key economic provisions. The union continued to ask for higher minimum pay scales and cost-of-living protection in addition to the wage and fringe benefit improvements offered by the company.

The company's announcement that it was imposing its final wage and benefits offer did not discuss the status of the non-economic provisions of the contract, such as work rules and job classifications. A company spokesman said no decision had been made on those contract terms, which under federal labor law can be modified by the company.

"The Post still wishes to enter into a new contract with the guild," said Boisfeuillet Jones Jr., counsel to the paper. He said the paper "will continue to respect the guild as the bargaining representative" of news, advertising and commercial workers.

Under the wage package implemented Monday, guild workers will get immediate raises ranging from $41 to $78 a week. In addition, part of the increase was made retroactive to the expiration of the old contract, producing lump-sum payments of as much as $2,800 for top-paid reporters and editors.

The package includes increases of up to 40 percent in pension benefits, expanded health insurance coverage, increased auto expense allowances and larger differentials for night work and certain positions. In addition, the newspaper agreed to set up a tax-deferred-savings plan, under which employes can contribute up to 2 1/2 percent of their salary and the company will match it.

The Post's action raises the minimum pay for reporters, editors and photographers from $557 a week to $616.50 a week. The company said the average pay of reporters will rise immediately to $43,500 a year.