A personal envoy from President Reagan met for almost two hours with Prime Minister Yasuhiro Nakasone here this morning in what appeared to be an emergency effort to defuse mounting tension over U.S.-Japanese trade.

Emerging from the meeting with National Security Council staff member Gaston J. Sigur, Nakasone told reporters they had discussed the full range of trade issues. "We talked a lot about how to collaborate," he said.

Sigur declined comment after the unusual Sunday meeting, conducted at Nakasone's office, saying the Japanese side would soon issue a statement.

Sigur arrived in Tokyo last night with Commerce Undersecretary Lionel H. Olmer in what appeared to be an 11th hour campaign to wrest concessions from Japan before new telecommunications regulations go into effect Monday.

Press reports here said Sigur had been expected to hand over a personal letter from Reagan urging Japan to take more forceful steps to further open its market. The United States hopes to reduce its trade deficit with Japan, which last year totaled $37 billion.

Reagan has formed close personal relations with Nakasone in a series of meetings during the past two years and is generally more sympathetic than Congress to Nakasone's claims that he is pushing as hard as he can.

In a television interview yesterday, Nakasone said that it is unavoidable that Japan's industry will make some sacrifices in the market-opening process.

Sigur, a Japan specialist on the NSC, is scheduled to meet with Japan's Foreign Minister Shintaro Abe later today.

Sigur is said to be on close personal terms with Nakasone and Susumu Nikaido, vice president of the ruling Liberal Democratic Party.

His visit came three days after the U.S. Senate, stung by news that Japan planned to increase auto exports to the United States by 450,000 vehicles a year, passed 92-to-0 a nonbinding resolution calling on Reagan to retaliate with tariffs or quotas against Japan unless it takes new steps to open its market.

Yesterday, members of the Japanese Cabinet met to discuss new market-opening measures that Nakasone has promised will be announced on April 9. But the meeting reached no conclusions.

In addition to steps in telecommunications, the government has promised action to facilitate foreign sales here of electronics, forestry products, medical equipment and pharmaceuticals.

Trade tension continued to be a major news story in Japan today, with newspapers and television news programs highlighting the negotiations.