The Japanese government announced today that it will reduce tariffs unilaterally on about 1,850 items by 20 percent or more next year in hopes of defusing U.S. anger over Japan's mammoth trade surpluses.

The government also declared itself ready to eliminate tariffs on all industrial products, subject to negotiations with its trading partners. Japan would expect concessions from them for such a step.

The plan, formally adopted at a two-hour meeting this morning of cabinet ministers and leaders of the ruling Liberal Democratic Party, will eliminate tariffs on 36 industrial and four agricultural items.

Those include many types of telecommunications and radio devices, products that U.S. companies hope to sell in greater numbers here. However, the plan leaves untouched other items of high interest to the United States, such as plywood and grapefruit.

Japanese officials tonight depicted the plan as a sweeping, positive signal of their government's good faith in addressing trade tensions with the United States and other trading partners. "We've accomplished very much," said a Foreign Ministry official.

A U.S. official in Tokyo, however, said that, while the plan did include many items of interest to U.S. companies, "tariffs are not the key issue." American trade negotiators maintain that nontariff barriers such as import procedures, standards and cartels are far more important as impediments to U.S. sales here.

Japanese and U.S. officials said Prime Minister Yasuhiro Nakasone was instrumental in lining up political support for the plan, which met strong opposition in powerful government ministries and the business community.

The package will affect about 80 percent of the approximately 2,300 product lines on which Japan imposes tarrifs.

About 1,790 items will be subject to an across-the-board 20 percent cut. Included are such things as paper products, electronics and film. However, the government will retain the right to restore the old tariffs on an item if the cuts damage domestic producers.

Among the items on which tariffs will be eliminated are types of telecommunications and radio equipment and batteries. These cuts, however, will not be subject to possible restoration. The government also plans to reduce tariffs irreversibly on 32 agricultural product lines.

Officials said they hope to implement the plan "as soon as possible" in 1986. Approval by the Diet, or parliament, is required, but with the backing of the cabinet and ruling party leaders, that is a virtual certainty.

Nakasone has promised that Japan will announce a package of nontariff market-opening measures in July. It is expected to cover such areas as import procedures and standards that products must meet to be sold in Japan.

All of these steps have come in response to a clamor that arose in Washington this spring over Japan's $37 billion trade surplus with the United States in 1984. Despite these and earlier measures, however, the surplus is continuing to grow; by some estimates it will reach $50 billion this year.

Japanese officials argue that, despite allegations abroad that Japan is a closed market, their country now has one of the lowest tariff schedules in the industrialized world.

But critics contend that Japan generally has lowered tariffs only if the affected local industry had became strongly competitive. (Today's package had comparatively little effect on farming, which in Japan is a highly inefficient and protected industry.)

In addition, Japan is charged with maintaining a vast collection of nontariff barriers.

Today's announcement specified that the cuts were being made to strengthen Japan's year-old campaign for a new round of international trade negotiations to revise the General Agreement on Tariffs and Trade. The United States also supports the new round of trade talks, but so far no date has been set.

Many of the agricultural products affected by today's plan are important to Japan's trading partners in Southeast Asia, which are mounting pressure of their own for access to the Japanese farm economy.