he nuclear-powered U.S. aircraft carrier Enterprise steamed into the American naval base at Sasebo in southwestern Japan today for its first call on a Japanese port in 15 years, amid noisy but small antinuclear demonstrations and tight police security.

The Japanese public as a whole, however, tacitly accepted the port call, apparently reflecting a greater willingness here for Japan to play a larger role in its 32-year-old military security alliance with the United States.

The visit, a liberty call for the Enterprise's 5,500 crew members, was regarded by many Japanese as emblematic of U.S. determination to strengthen its military presence in the Western Pacific to offset a rapid Soviet naval buildup in the region.

The arrival of the Enterprise at Sasebo in January 1968, at the height of the Vietnam War, touched off bloody confrontations between thousands of antiwar protesters and police that left 500 injured. In contrast, 3,000 riot police today easily contained a crowd of several hundred snake-dancing political activists who shouted anti-American slogans near the gates of the U.S. naval base.

The 75,700-ton Enterprise was escorted to its berth by two dozen Japanese coast guard patrol boats manned by helmeted martial arts experts wearing fireproof uniforms and armed with riot shields and nightsticks. The flotilla kept at bay a handful of fishing boats chartered by demonstrators belonging to Japan's small radical leftist groups. Six demonstrators reportedly were arrested when they tried to hurl a smoke bomb at one of the patrol vessels.

An estimated 10,000 persons took part meanwhile in a rally organized by Japan's Socialist Party to protest what a senior party official said was an attempt "to strengthen the U.S. Navy base at Sasebo as an obvious part of the Reagan administration's anti-Soviet strategy. The arrival of the Enterprise will lead to the introduction of more U.S. aircraft carriers and nuclear submarines in a bid to acclimatize the Japanese public to nuclear weapons."

The Enterprise carries about 90 jet fighter planes and attack aircraft capable of being armed with nuclear bombs, which are believed normally to be stored in the ship's hold. Tokyo maintains a policy which, in principle, forbids the production, possession and introduction into Japan of nuclear weapons.

Public statements by former American officials that U.S. warships have routinely carried nuclear weapons into Japanese ports have been the focus of intermittent controversy here in recent years. But, political analysts here point out, the response among the Japanese press and public to such alleged policy breaches has become increasingly low-key as Japan has embarked on a gradual defense buildup to help counter Soviet military power in the Far East.

According to U.S. intelligence estimates, the Soviets have more than 150 submarines operating in the Pacific and have stationed a growing number of Backfire bombers, MiG 21 fighters and SS20 intermediate ballistic missiles in the area. They are also reported to have substantially beefed up the installation they inherited from the United States at Cam Ranh Bay in Vietnam.

To offset the Soviet presence, Pentagon officials recently have indicated that the United States will begin shifting the focus of its own naval buildup from the Indian Ocean to the Western Pacific. Japanese defense analysts expect that the recently overhauled battleship New Jersey, now capable of carrying Tomahawk cruise missles, and the newly built nuclear-powered aircraft carrier Carl Vinson may soon follow the Enterprise in making port calls here.

Since coming to office Nov. 26, Prime Minister Yasuhiro Nakasone repeatedly has stressed his determination to strengthen Japan's military alliance with the United States. His hawkish stand on defense, including a statement made in Washington in January that Japan should become "an unsinkable aircraft carrier" against Soviet Backfire bombers, has left him open to widespread charges here that he is attempting to move far too fast in the military sphere.

According to Masaru Ogawa, a political commentator, however, "there are signs of a more realistic approach toward the U.S.-Japan security arrangement being taken by the people than a decade and a half ago." For example, Washington's decision last year to station a wing of 40 F16 fighter-bombers at Misawa in northern Japan beginning in 1985 failed to raise the public flap here that many observers had anticipated.

The Enterprise came to Sasebo for a five-day liberty visit following its participation in annual joint military exercises between U.S. and South Korean forces. Communist North Korea blasted the port call, charging it "makes Japan a base for American imperialism to invade Asia and damages Japanese independence."

In Sasebo, however, most citizens appeared to welcome the "Enpura," the Japanese phonetic shorthand for the Enterprise. Hard hit by the slumping fortunes of the local shipbuilding industry, the city receives about $45 million each year from the United States for repairs, provisioning and support facilities for American naval vessels. Tatsuo Sato, a local business official, was quoted as saying, "The citizens here are too hungry to worry about whether the Enpura visit is good or bad."