The main streets of Shin-Okubo — Tokyo’s Koreatown — are lined with smoky barbecue restaurants and over-lit cosmetics emporiums. Staircases lead down to basement music venues and up to hidden drinking holes.

Japanese once thronged the neighborhood, which is home to many ethnic Koreans and known for its fiery food and late nights. But in recent months the crowds have thinned, replaced by anti-Korean protesters who have turned Shin-Okubo into a rough barometer of deteriorating Japan-Korea relations.

On occasional weekends this year, megaphone-wielding demonstrators have taken to the streets, telling the Koreans to “go home or die.” They’ve threatened to “flatten this neighborhood” and build a gas chamber in its place. The Koreans say that they — and the police — have little recourse against the threats, because Japan is one of the few democracies that don’t restrict hate speech.

The protesters are a small but noisy lot, and their strident anti-Korean stance is viewed with contempt by most Japanese. But the demonstrations have caused damage nonetheless, not only disrupting a neighborhood, but also providing kindling for the South Korean media, which portray the behavior as a frightening norm, not an extreme.

In that way, the demonstrations have helped widen the divide between the United States’ two closest Asian allies, countries that have squabbled for decades but now increasingly see themselves as archrivals. As if to highlight the point, South Korean President Park Geun-hye said recently that she would be open to a summit with North Korean leader Kim Jong Un — but not with Prime Minister Shinzo Abe, unless Japan changes its behavior.

The animosities on the Korean side are rooted in Japan’s brutal 1910-1945 occupation of the Korean Peninsula. But they are kept alive by mutual distrust. Koreans, both North and South, say they are owed a more sincere apology for past ills, including the wartime use of female sex slaves for soldiers. Japanese say Koreans are just holding a grudge and ignoring 70 years of model behavior under the pacifist Constitution.

Opinions in Japan and South Korea agaist each other have deteriorated drastically, and to noxious effect: Last month, Park was criticized at home for wearing Asics, a Japanese brand, to a baseball game.

Only a year ago, the Shin-Okubo cosmetics store Popberry sold products labeled in Hangul, the Korean alphabet. But no longer.

“It still says ‘Made in Korea’ on the back,” owner Ryu Eun-sook, a Korean immigrant, said, pointing to a product. “But now there are no Hangul letters.” Some of her wholesale clients were worried Hangul products would no longer sell, she said.

The most recent protests in Shin-Okubo took place in early September — they were more frequent in the first half of the year — and residents here say they hope they’ve weathered the worst.

But when the weekend protests do happen, they are ugly affairs — and residents say they should be forcibly stopped. They are organized by a small number of citizens’ groups, most notably Zaitokukai, which claims nearly 14,000 members nationwide. Protests have also been held in other cities, including Osaka.

Zaitokukai’s members aren’t just a collection of the down and out, according to experts and media reports. Membership skews young and male but also includes well-coiffed housewives. They organize their meet-ups on the Web, and they campaign against the permanent residency status given to Japan’s half-million ethnic Koreans, many of them descendants of laborers brought here during the colonial occupation. Zaitokukai’s head office declined to let members be interviewed, citing a schedule conflict. It also declined to answer questions by email.

The group’s demonstrations in Shin-Okubo began about a year ago, about the time Abe was elected, according to residents’ accounts. But some experts and politicians say their emergence coincides with broader factors: Japan’s slackened economic might and a growing sense among right-wingers that the country must find new ways to show its strength.

“Japan is right now at a crisis point,” said Yoshifu Arita, a lawmaker who is campaigning for new laws to regulate hate speech. “A situation like this — people getting so publicly hostile — never happened in the seven decades after the war until now.”

In practice, groups like Zaitokukai campaign with the purpose to intimidate. They march through streets holding Rising Sun battle flags of the Imperial era and placards that tell Koreans to go “hang themselves.” Police try to keep the protesters to one side of the street, allowing a passageway for cars.

In 1995, Japan did accede to the United Nations convention to eliminate racial discrimination, including hate speech, but the Diet has not passed legislation to enforce that treaty commitment.

Its reluctance, experts and politicians say, stems from a separate war-era legacy — the wholesale suppression of anti-government dissent.

Japan created free-speech laws to prevent a repetition of that censorship, and many still oppose the idea of regulating speech, said Kenta Yamada, a media law professor at Senshu University. The government’s hope, Yamada said, is to reduce hate speech with education and enlightenment, not with new laws.

Abe himself has called the protests “extremely unfortunate” and said that true Japanese people “must be polite, generous and humble.”

In Shin-Okubo, the downturn in Korea-Japan relations — and the rise of demonstrators — is viewed above all with sadness. Some restaurateurs say business hasn’t suffered. But shopkeepers like Ryu say it’s down sharply.

The first time Ryu heard the anti-Korean protesters marching on a main street below her office, she was angry enough to think about heaving wooden furniture through her fourth-floor window. But lately, she’s stayed calmer and has tried to think about the 25 Japanese staff members she employs.

“I know they are kind-hearted,” said Ryu, who moved to Japan 18 years ago and obtained legal residency.

Some ethnic Koreans get calls from friends back home after protests.

“They say, ‘Are you OK?’ ” said Lee See-hyun, an employee at a record store selling Korean pop music. “I tell them I’m fine. I don’t believe most Japanese want us to go home.”

Since early this year, a new movement has emerged to reinforce this view. When the ultranationalists gather in Shin-Okubo, another group — a loosely organized mix of Japanese citizens and activists — gathers on the other side of the street. They jeer at the ultranationalists and use air horns to drown out their chants.

“You’re a Japanese shame!” one woman shouted to them during a recent gathering.

“Stop the racism!” another said.

The clashes have sometimes turned violent. In July, several members from both groups were arrested after they began spitting at one another and trading punches.

The counterprotesters have barely been mentioned in the South Korean media, but for many ethnic Koreans living in Shin-Okubo, they more accurately reflect Japanese sentiment. In some cases, the hundred or so anti-Korean protesters have been well outnumbered, according to videos of the protests, and the police try to keep the groups separate.

Ryu has seen this, too, from her window. The clashes make her uneasy, she said, but added, “Somebody is standing up for us when we can’t. So yes, they make me appreciative.”