GENEVA, DEC. 2 -- Japan, caught between its obligations under international trade rules and political pressure at home, tried today to block parts of an impartial panel's findings that its restrictions on farm imports are illegal.

Faced with a U.S. threat of retaliation and opposition from 12 other countries, Japan asked for another day to reassess its position.

The decision is one of the toughest to arise in the early days of the government of Prime Minister Noburo Takeshita, who warned his countrymen last week that they will have to "forbear and endure" unpopular measures to reduce trade frictions with other countries.

Politically powerful farm groups have staged three large rallies in recent weeks demanding that Japan ignore the decision of a panel formed under the 95-nation compact that polices world trade, the General Agreement on Tariffs and Trade (GATT).

Japan had offered to accept all but two parts of the panel's decision. It said it would agree to a finding against its restrictions on imports of 10 groups of agricultural products if it could block the decision on two other groups -- starches and dairy products -- that are the most politically sensitive.

The finding, which U.S. trade officials call the most significant in the 40-year history of GATT, came on a complaint by the United States that was filed in October 1986 after three years of unsuccessful negotiations. The products, all processed foods, include canned corned beef, canned pineapples, tomato catsup and fruit and vegetable juices.

Under GATT rules, any country can block adoption of a panel decision. But U.S. Trade Representative Clayton K. Yeutter said he warned Japanese Foreign Minister Sosuke Uno here earlier this week that a refusal to accept the GATT decision would bring "a harsh reaction from the United States."

He said he also told him that Japan would earn the "animosity and castigation of the entire world trading system" if it refused to accept the adverse finding.

Japan already is under attack by the 12-nation European Community, which accuses it of taking the benefits of the world trading system without living up to its obligations.

On Thursday, the Takeshita government is expected to accept the entire GATT panel finding reluctantly. According to reports here, the government will use the strong opposition it met with here today to persuade farm lobbies that Japan has no choice but to give in to GATT.

In accepting the decision, Japan can either end the trade restrictions or, if that is too politically difficult, pay compensation equal to the lost sales. It is unclear how much that will amount to, although even with the restrictions the U.S. exports $88 million of those products to Japan.

The ruling is seen by U.S. farm groups as a precedent for ending other Japanese barriers to imports of farm products, including items not covered by the complaint, such as beef, citrus fruits and rice. The Japanese also recognize this, which is one reason farm groups there have mounted such a vigorous campaign against accepting the GATT decision.

But U.S. trade officials believe the importance of the finding goes beyond farm products. Ending Japan's high subsidies for domestic agricultural products and its tight barriers is seen as critical if that society is to be transformed so that it depends on domestic demand rather than overseas sales.

By protecting its farmers, the Japanese government allows the price of food to go so high that it greatly reduces the funds available for other consumer products. Using so much valuable property for small, inefficient farms, furthermore, is a cause of skyrocketing land prices, which make it impossible for Japanese to afford the kind of housing that people in other rich industrialized nations enjoy.

But Japanese farmers form a potent lobby and are influential in the ruling Liberal Democratic Party. Mitsugu Horiuchi, head of the 5 million-member Central Union of Agricultural Cooperatives, told a rally of 1,000 farmers in Tokyo Tuesday that Japan "can by no means accept" the GATT finding, the Associated Press reported.

The United States was backed in the debate by Argentina, Australia, Uruguay, Chile, Hungary, Brazil and the Philippines, which spoke for all six members of the Association of Southeast Asian Nations. No other nation supported Japan's position.