Sen. John C. Danforth (R-Mo.) today called on Japan to increase dramatically its purchases from abroad, saying failure to do so could start the breakdown of the international trading system on which its present prosperity is built.

"Now is the time for a self-satisfied Japan-as-Number-One generation to learn that there's more to leadership than flooding the world with Walkmen and VCRs," he said in a speech to the Foreign Correspondents Club of Japan.

"No other nation contributes so little to the open trading system of the world, in proportion to what it gains," he said. " . . . Japan is a great country. It should begin to act like one."

Danforth, visiting Japan with five other U.S. senators, said Japanese leaders agree on the need for change. But he said he was "fairly pessimistic" that Japan would be able to make the needed changes.

He said Japan's estimated $50 billion trade surplus with the United States in 1985 "fosters a resentment and growing hostility that threatens political consequences as well. . . . In the United States, public support for free trade has collapsed."

He called on Japan to continue limiting automobile exports to the United States after a program that holds them to 2.3 million vehicles a year expires at the end of March. Japanese officials told him they had made no decision yet, he said.

Danforth said efforts so far to solve the trade imbalance have been "mostly talk." He said he would withhold judgment on negotiations on telecommunications, which the United States and Japan concluded last week and declared a success. Danforth, sponsor of a bill seeking reciprocity in the telecommunications trade, said there are no tangible results yet. "I've learned from experience never to claim victory before the numbers are in," he said.

Reagan administration officials say the telecommunications talks already have resulted in major new sales for U.S. companies.

Danforth made no specific proposals for how Japan should go about increasing its imports. But he praised proposals to step up Japan's domestic economy, which many analysts feel would draw in more foreign products.

He said a reasonable goal for Japan would be to double its imports of manufactured goods and processed agricultural products in three to five years. He said that manufactured goods account for only 25 percent of Japan's total imports compared with 64 percent for the United States.

Danforth and the delegation arrived in Japan Friday. They have conferred with Prime Minister Yasuhiro Nakasone and other Japanese leaders.

Because of his influence on trade issues, Danforth and his remarks are being monitored closely by the Japanese as a barometer of congressional sentiment for action to curb their sales to the United States.

Japanese officials maintain that their country is already an essentially open market. But they say foreign companies often fail in Japan because of poor product quality and marketing aggressiveness.

Concerning charges that Japan is flooding the world with its products, they argue that exports occur only if foreign consumers want to buy Japanese products.

Danforth said the United States could help to brighten the international trade picture by controlling the federal budget deficit and improving its product competitiveness. But he said that, under the current system, meeting those goals still would have little effect because Japanese barriers would keep them out.

"Japan imports only what it cannot produce itself," he said. " . . . The world trading system cannot long function when its second-largest economy abdicates its responsibilities. Because of its size, a Japanese economy that formally or informally remains closed threatens the openness of the system itself."