Japan and the United States, unable to meet an April 1 deadline, have agreed to keep talking about two key points in new regulations taking effect today that are to make sweeping changes in the telecommunications industry here.

Prime Minister Yasuhiro Nakasone told a personal envoy from President Reagan yesterday that Japan would move quickly to solve remaining issues, according to Japanese accounts of the meeting, but he did not say that Japan would make concessions.

The envoy, National Security Council member Gaston Sigur, reportedly told Nakasone that Congress was likely to pass protectionist legislation aimed at Japan unless it gave more ground on telecommunications.

A Foreign Ministry official said Sigur made the point that "to stop this dangerous protectionism movement, we need more action, quickly and powerfully" and referred to a bill that could be considered this week in Washington.

The Japanese interpreted this to refer to a bill putting teeth in a Senate resolution passed 92 to 0 on Thursday calling for retaliation if Japan did not open its market to American goods.

About 200 pages of new Japanese government regulations went into effect today, ending a state monopoly over telecommunications services held by Nippon Telegraph and Telephone since 1952. The field is now officially open to competition by Japanese and foreign companies.

In recent weeks, fine print in the regulations have become the focus of mounting trade tension between the two countries. The rules were subject to intense negotiations with the United States, which contended that Japan was drafting them to discriminate against foreign firms.

The two governments had hoped to settle all questions by today, when Japanese law requires that they go into effect. But the negotiations, taken by the American side as a barometer of Japan's overall intentions to reduce its trade surplus with the United States, have moved slowly.

Japan contends that it already has given ground generously and is unfairly being depicted by Congress as the sole cause of the surplus, which reached $37 billion last year and is expected to grow.

After handing over a letter from Reagan, Sigur told Nakasone that the United States wants stronger assurances that Japanese officials, in making future revisions of the regulations, will solicit foreign firms' views and give them serious consideration.

On the first of the two points that the Japanese identify as still disputed, officials said they have told the United States that they will announce any future plans to revise regulations in advance. They also have set up study groups in which foreign-owned companies incorporated in Japan will be represented, and they said they are willing to discuss the issue further.

Sigur reportedly also pressed Japan to adopt U.S. views on technical standards, Japanese officials said. Washington argues that new equipment should be judged only by the question of whether its use would harm the communications network here, an approach akin to that in the United States.

Japan, however, holds that equipment must meet specific standards in about 30 separate categories, such as voice quality and signal strength. The Americans suggest this would tend to limit market access and distort the market.

Japanese negotiators earlier had agreed to continue talking about this for another 60 days. But according to a Foreign Ministry offficial, in yesterday's meeting "the prime minister said that situation is so tense that he will give instructions to solve this before 60 days."

Sigur, who was accompanied to Tokyo by Commerce Undersecretary Lionel Olmer, later met with Foreign Minister Shintaro Abe and Minister of Posts and Telecommunications Megumu Sato. He was expected to leave for Washington today but so far has declined comment on the talks.

The broad outlines of the new regulations have been known for weeks. NTT will lose its monopoly. New companies will be free to offer competing telephone service, which is expected to focus on long-distance service in the high-density Tokyo-Osaka corridor.

Rival companies also will be allowed to set up what are known here as "value-added networks," computer switching services using leased lines that can link otherwise incompatible computers.

During the talks, the U.S. side persuaded Japan to simplify registration requirements for the computer networks. Japan also agreed to accept foreign test data in certifying new equipment.

However, many Americans say the Japanese have preserved an overregulated system that will give officials here the power to discriminate against foreign suppliers and make decisions behind their back.

In January, after a meeting with Reagan in Los Angeles, Nakasone instructed his Cabinet to come up with market-opening measures in telecommunications, electronics, forestry products and medical products. Nakasone told Sigur that on April 9 the government will publish a list of immediate steps in these fields.