At his first press conference, Japan's Prime Minister Yasuhiro Nakasone today defended his controversial Cabinet choices and reinforced his apparent determination not to follow the self-effacing style of previous Japanese leaders.

The new prime minister said President Reagan had telephoned earlier today to offer congratulations. Nakasone said he told Reagan of his desire to visit the United States in the very near future.

In his meeting with reporters, Nakasone strongly denied that indicted former prime minister Kakuei Tanaka, currently on trial in the Lockheed bribery case, had influenced his selection of Tanaka followers to several key Cabinet posts announced during his first day in office yesterday.

He described his choices as men who would fit the mold of dynamic leadership required to deal effectively with pressing national issues, including the need to revive Japan's sluggish economy, blunt the growing protectionist backlash against Japanese goods abroad and shoulder a greater burden for the country's defense.

Nakasone's selection of traditional faction supporters was viewed by the press here as a retreat from his campaign promise to move away from a system of politicking that has been dominated by backroom dealings among competing faction leaders of the Liberal Democratic Party, which has ruled without interruption for 27 years.

In two days in office, the new prime minister has suggested that Japan may have to abandon its policy of holding defense spending to less than 1 percent of the country's gross national product to meet its responsibilities as a member of the Western alliance, saying that "others are making sacrifices, so Japan must try harder to carry its share of the load." He also has spoken of a possible need to rewrite Article 9 of Japan's postwar constitution, which renounces war as an instrument of national policy, although he told reporters today that his administration would not be directly involved in such a revision.

Nakasone has stressed the need to improve ties with Washington that have been badly strained by huge U.S. deficits on trade with Japan, which are expect to reach a record $20 billion this year. And he has acknowledged that Japan's modest military budgets have not been in keeping with this nation's formidable economic clout.

The forceful, flamboyant Nakasone, known for his hawkish views on national defense and with a reputation for political opportunism, seems likely to upset the status quo in a society traditionally governed by the concepts of public harmony and consensus.

A dramatic public speaker who displays a populist flair, Nakasone, 64, for years has been favored by Japanese voters for the premiership. His popularity grew as voters increasingly became dissatisfied with the bland, unobtrusive style of the aging politicians who lead the conservative Liberal Democratic Party.

In remarks rare to Japan's typically unassuming style of politics, Nakasone had declared earlier this week: "Today, life has become increasingly complex and we face many problems on the international front. Uncertainty and anxiety have crept into Japanese life and to break through these difficulties stronger political leadership with more clearly defined policies is needed."

Yet now that Nakasone has emerged from the gray political landscape to assume Japan's highest political office, Japanese appear beset by conflicting emotions over where, and by what means, the new prime minister will lead this peaceful, prosperous nation of 117 million.

Newspaper editorials today charged that the new premier, in his first act of leadership, had betrayed the public trust by appointing Tanaka's proteges to his Cabinet. The Mainichi, a major national paper, warned that Nakasone's popularity, "will not rise unless he takes the bull by the horns in reforming the antiquated style of the Liberal Democratic Party."

The controversy over Nakasone, Japanese political observers say, comes at a time when Japanese are seeking stability and change simultaneously. This seeming paradox explains why voters continue to back the Liberal Democratic Party at the polls while sharply criticizing the party for its seemingly endless factional rivalries and often murky policy preferences.

Since the business-oriented Liberal Democratic Party came to power almost three decades ago, Japanese have experienced a dramatic improvement in their living standards and have shied away from political changes that would risk giving the country's badly splintered, leftist opposition parties a major role in the government.

At the same time, they suffer from a sort of free-floating anxiety about the future. Japan's economy, although still buoyant by world standards, is growing at an annual rate less than half that of the booming days of the early 1970s. Facing massive, chronic budget deficits, the country is being urged by the United States to assume a greater and more costly defense role, including policing sea lanes and airspace 1,000 miles from its borders.

These new pressures have prompted many Japanese to look for a more assertive brand of conservative politics. Appealing to this sentiment, Nakasone promised to break through the logjams in the labyrinthine bureaucracy that have made progress on important public issues painfully slow.