Japan's Navy today embarked on its first multinational training exercise against a background of domestic protest that is only a faint echo of the antimilitary turbulence of past years.

Ships and planes of the Maritime Self-Defense Force joined naval forces of four other Pacific countries -- the United States, Canada, Australia and New Zealand -- in the central Pacific, ending Japan's postwar tradition of abstainng from collective defense operations.

Japan has shared military operations for more than a decade with the United States, but for domestic political reasons it stayed out of exercises involving other nations.

Its participation in this year's central Pacific exercise touched off a modest revival of the hostility toward military preparations that marked the late 1950s and 1960s.

Left-wing parties and labor unions denounced the participation as a new violation of Japan's antiwar constitution and the small but violent underground chimed in with sporadic sabotage.

About 5,500 demonstrators organized by the Socialist Party and affiliated labor unions protested Sunday at the naval port of Yokosuka and today national railway engineers expressed their disapproval by delaying more than a hundred trains.

Last week, a radical group claimed responsibility for cutting communication cables at military installations in another protest of Japan's joining the naval exercises off Hawaii.

But these and other minor incidents at bases were far less disruptive than the massive demonstrations that once confronted effort to expand Japan's military preparations and to continue the U.S.-Japanese security pact.

Those protests reached a peak in the summer of 1960 when Japan admitted it could not guarantee security for a visit of the late president Eisenhower. Protests continued throughout the 1960s but gradually subsided.

It would have been unthinkable in those days for Japan to send two destroyers, eight antisubmarine patrol aircraft, and 700 seamen to join a four-nation force as it did today.

Japanese observers regard the light protest this year as reflecting a slow but steady change in Japanese attitudes toward defense, a change that gradually gives military leaders more maneuvering room to stretch the words of the constitution to cover new types of military activity.

"Compared to 10 years ago, the Japanese people's attitude toward things like this has changed," said Ganri Yamashita, a member of parliament and former head of the Japanese Defense Agency. "Their understanding for these things has improved."

The total force for the 20-day exercise involves 41 ships, 200 aircraft, and 20,000 sailors and airmen in joint operations to test combat readiness, according to U.S. military officials here.

To deflect the opposition, the Japanese Defense Agency has insisted that its naval forces are merely participating in a "training exercise" to improve its sailors' skills, as authorized by law. This exercise, it asserts, is not a collective security operation aimed at a potential enemy, which would be a clear violation of Japan's constitution.

Socialists, communists and leftist labor union leaders scoff at this explanation as semantic legerdemain designed to obscure the fact that Japan has taken another small step in expanding military operations, this time abroad.

They see it as a prelude to further militarization linking Japan with other Asian noncommunist nations.

In the past year, the United States and Japan have stepped up their joint exercises and recently conducted their first coordinated air defense training.

Since the Soviet invation of Afghanistan, the United States has increased its pressure on Japan to strengthen its defenses and has suggested that it take on more responsibility for defending its commercial shipping lanes through Southeast Asia.