A top Reagan administration trade official yesterday accused Japan of failing to implement six-month-old promises to make it easier for American goods to be sold there and said this country's patience is wearing thin.

"It is essential that Japan live up to its previous commitments given to the Reagan administration to liberalize foreign access to its economy. Neither the letter nor the spirit of the May 28 liberalization measure has been accomplished," Deputy U.S. Trade Representative David Macdonald said in releasing a 75-page report to Congress citing Japanese barriers to American exports.

He said Japanese liberalization measures, which had been moving at "a snail's pace," sped up to "a turtle's pace" with the May 28 promise to ease trade restrictions. But they will have to accelerate to "a hare's pace" because "the constituency for free trade in the United States has almost totally eroded," Macdonald said. "If we are to rebuild it, quick action by the Japanese on their own announced measures, as well as agricultural liberalization, is vital."

"Whether it realizes it or not, the government of Japan is in a race against time," Macdonald continued.

The American trade official's warning, one of the stiffest in a long series issued by the Reagan administration against Japan, came shortly after U.S. negotiators failed to make headway in a series of talks on such key trade issues as reducing barriers to the import of tobacco, beef and citrus to the Japanese market.

Macdonald issued his statement just two weeks before ministerial-level talks involving 88 of the world's major trading nations are to begin in Geneva. The Reagan administration's free-trade views are expected to meet widespread opposition at those General Agreement on Tariffs and Trade (GATT) talks.

The head of the European Community's delegation to the United States, Sir Roy Denman, yesterday resisted demands by Agriculture Secretary John R. Block that Europe end its subsidies of farm exports or face the dumping of $3 billion worth of surplus American dairy products on the open market. Block called his threat "a very effective 2-by-4" to be used against the Europeans.

Pointing out that the European Community has a $7 billion agricultural trade deficit with the United States, Denman told reporters that trade disputes have to be discussed "rationally and in the spirit of cooperation" if the world trade system is to survive.

Macdonald, however, was far more circumspect than the agriculture secretary in detailing possible Reagan administration moves to force the Japanese to ease their trade restrictions.

The trade official said that Japan's failure to live up to its promises in May will "permeate" its entire trade relation with the United States. The next major issue on the table is an expected Reagan administration request that Japan extend for a third year its limits on auto shipments to the United States, and Macdonald raised the possibility of further demands on Japan to reduce the number of cars below the present annual limit of 1.68 million.

Macdonald said he recognizes the Japanese government's domestic problems in opening its markets to foreign goods -- especially in the field of agriculture, because farmers there have heavy political clout. But he said Tokyo must recognize the political pressures on the administration, which "are now becoming so strong that we must take action where we feel our legal rights are being infringed on by Japanese government actions."