Amid threats from sound trucks belonging to Japan's right-wing extremists that he would be attacked, Satoshi Tomizawa decided last week to batten down the doors of the new two-story concrete and steel building he operates here.

Tomizawa is the manager of the Eastern Hokkaido Japan-Soviet Friendship and Trade Pavilion, one of five such structures that have sprouted on the Hokkaido landscape in recent years to cater to Soviet officials and business representatives who ply a booming trade with Japan's second-largest island.

The state of siege was touched off by Prime Minister Zenko Suzuki's trip here last week to campaign for the return of the four islands seized by the Soviet Union at the end of World War II, and reflects the deep split in local sentiments on an issue that has raised strong emotions throughout Japan.

The noisy members of Japan's right-wing organizations roared through the streets of Kushiro, Japan's largest fishing port, on their way to Nemuro, about 60 miles away, where Suzuki's military helicopter touched down Sept. 10 after his unprecedented inspection tour of the Soviet-held islands.

Dressed in olive-drab fatigues and with sound trucks blasting out prewar martial hymns, the rightists massed to object to the government's stand on the reversion issue, which they charge has been too soft on the Soviet Union. This was despite the fact that Tokyo has now taken a tough new public posture.

Tight security kept the rightists far from Suzuki's party. But they did succeed in registering a loud protest to local business groups, who they charge are selling out Japan's national interests in favor of close and profitable ties with the Soviets.

The Suzuki visit capped a nationwide drive that has drummed up overwhelming public support in recent months for the return of the islands, off the northeastern tip of Hokkaido and immediately southwest of the Soviet-held Kurile Islands.

In bringing pledges of economic aid for local areas here, however, the Suzuki government appeared to acknowledge the serious breach in public opinion in Hokkaido between groups favoring friendly ties with the Soviets and those insisting that Japan should get the islands back no matter what the costs.

Pro-Soviet attitudes are strong among businessmen here who stand to lose timber contracts and fishing concessions from the Soviet Union should they too openly endorse Tokyo's policies. Those sentiments are mirrored in the support for organizations such as Tomizawa's.

Tomizawa ushered recent visitors into a spacious reception room festooned with Japanese and Soviet flags and crisscrossed with party streamers. Here, under the steady gaze of portraits of Lenin and Soviet President Leonid Brezhnev, his organization entertains several hundred visiting Soviet dignitaries each year and hands out pro-Soviet literature to local Japanese.

Kushiro's Soviet Friendship Pavilion, like the ones in Hokkaido's other major port cities, was built at Soviet prompting. A hundred local businesses raised $150,000 to construct the building .

Fishing dominates the economy of this city of 206,000 and it slumped badly in 1976 when Moscow proclaimed a 200-mile economic sea zone, cutting Kushiro's fishermen out of traditional fishing grounds in Soviet waters. Since then, they gradually have been allowed back in.

Yatsushiro Hirano, a Kushiro city official, explained: "Kushiro is the biggest base of Japan's fishing force and the Soviet Union is our biggest partner. All fisheries activities must be coordinated with the Soviets, so we can't take up the reversion movement without taking account of the Soviet response."

Last December, Kushiro's City Council passed a resolution endorsing the return of the four islands. But as in most other major economic centers in Hokkaido, the weighty economic interests at stake make enthusiasm for the movement hard to muster. "There is a feeling," Hirano said, "that things must be done in a friendly way."

In nearby Nemuro, however, emotions on the reversion issue run much deeper. Many of the surviving 17,000 islanders who were expelled by the Soviets in 1945 live there and, on Japanese maps, the four islands are part of the city's township.

Mayor Isao Terashima said that Moscow's imposition of the 200-mile limit and its hard line on the islands issue have seriously damaged Nemuro's economy because of its heavy reliance on fishing close to the disputed territories.

Because of the depth of local sentiment demanding reversion, Nemuro's fishermen are treated more harshly by Soviet patrol boats. "The simple fact," Terashima said, "is that the northern islands issue has carried over from World War II and the war is not yet over in Nemuro."

Moscow blasted Suzuki's trip here last week, with Tass, the official Soviet press agency, calling it "provocative." The Kremlin has rejected negotiations on the islands' future and it claims that Japanese public opinion favoring reversion has been artificially created by Tokyo.

Pravda, the official Soviet Communist Party newspaper, said this week that Suzuki had failed to recognize Hokkaido residents' sentiment favoring good neighborliness and the development of Japanese-Soviet economic ties.

The Soviet Union recently has stepped up efforts to drive the wedge deeper between Japanese by issuing dire warnings against Tokyo's "unfriendly acts." At the same time, Moscow has started to allow fishermen to gather seaweed near one of the disputed islands for the first time in five years.

Tokyo mounted its recent campaign at least in part because of growing concern over what some officials view as Soviet attempts to "Finlandize" Hokkaido -- bring it under direct Soviet influence -- and leaders of Japan's ruling Liberal Democrats have called Soviet efforts here an "indirect invasion."

One Japanese official in Tokyo said the Soviets think that "if they can divide our public opinion, they can freely pursue their policies." But, he added, "friendship doesn't mean anything to the Soviets, and really good relations will come only after the return of the territories."

In an apparent attempt to drive this point home to Moscow, Suzuki this week made a two-day official visit to Okinawa, which reverted to Japanese control in 1972 after 27 years of postwar U.S. occupation.