TOKYO — The police said on Monday that eight people were arrested after fistfights broke out between ultranationalist Japanese shouting anti-Korean slogans and counterprotesters on the streets of Tokyo, where public violence is rare.

Among those arrested was Makoto Takada, 41, better known by his pseudonym Makoto Sakurai, the leader of an anti-foreign ultraconservative group that has recently gained attention for staging repeated demonstrations in an ethnically Korean neighborhood. During those marches, members of the group have waved Japanese war flags and shouted slogans like “Kill Korean residents,” raising calls for the creation of anti-hate-speech laws.

The group is known in Japan as the Zaitokukai, but its full name translates as the Citizens Group That Will Not Forgive Privileges for Koreans in Japan. Claiming 12,000 members, it is the most visible of a new batch of ultranationalist groups known as the Net right because they use the Internet to organize. Though relatively small in size, they have gained public attention in recent years as the virulent side of Japan’s anxiety over its declining economic and political position in Asia.

The fights, in Tokyo’s Shinjuku district, began on Sunday afternoon when about 200 Zaitokukai members, led by Mr. Takada, assembled to stage a demonstration in a Korean neighborhood, Okubo, the police said. The would-be marchers were intercepted by about 350 members of a counterprotest group shouting “Racists go home,” the police said.