The Washington Post yesterday declared an impasse after 16 months of bargaining with the Newspaper Guild and said it will implement major portions of its economic proposals to the union, including retroactive pay increases.

"The guild is demanding substantially more than our final offer and there is no contract settlement in sight," said John B. Kuhns, vice president for personnel at The Post. "We have no more movement in our position, and thus the parties are at an impasse at this time. However, it is our sincere hope that the guild will eventually change its position and we can sign a contract."

Negotiators for the guild, which represents 1,400 news, editorial, advertising, circulation and business employes of the paper, disputed the company's position. Chief negotiator Sandi Polaski said the company declared the impasse while the two sides were in the midst of an exchange of proposals at a bargaining session yesterday afternoon. She said the guild had promised in the session to modify its position by next week in response to a new company proposal made last Thursday, and had submitted a proposal of its own yesterday in another area.

"We are not at an impasse," Polaski said. "The logjam seemed like it was starting to break up last week." She said she had just finished congratulating the company negotiators for beginning to move when they caucused, declared an impasse and walked out of the talks.

The two sides have been negotiating to replace a contract that expired July 9, 1986.

Kuhns said the economic package offered to guild employes, including salary increases, is worth between 4 and 5 percent more a year for three years, which "compares very favorably not only with guild settlements elsewhere, but also with union settlements generally in this area and throughout the country. It is comparable to settlements we have reached with other unions at The Post."

But Polaski said the newspaper's offer continues a two-tier wage system, in which newer employes are paid on a lower scale, that the union contends discriminates against minorities and women. It also will force workers to pay part of their medical costs now covered by insurance, she said.