The alleged kingpin of the U.S.-based arm of the Russian mafia was arrested here this morning by FBI agents on extortion charges, opening what law enforcement officials said was a major assault on the growing Russian organized crime presence in the United States.

Vyacheslav Kirillovich Ivankov, 55, and five of his associates were picked up in the "Little Odessa" section of Brooklyn, where the bulk of the city's Russian emigre population lives. Three others remain at large.

The arrest follows a long and difficult investigation by the FBI in cooperation with the Royal Canadian Mounted Police and the Russian Ministry of the Interior. Efforts at gathering evidence were repeatedly hampered by Ivankov's elusiveness and the impenetrability of his organization.

As he was led from FBI headquarters in downtown Manhattan, Ivankov, a stocky and well-groomed man known in the Russian community for his urbanity and charm, kicked and spat at photographers. He was arraigned later at U.S. District Court in Brooklyn.

"This is not a huge blow from the standpoint on the impact of the day-to-day operations of the Russian mob tomorrow or week from now," said James Kallestrom, assistant director of the New York FBI office. "We didn't go out and arrest 300 people and seize property. But it's a shot across their bow. We're telling them that things are different here, that we're not going to put up with them and they are not going to be able to pay their way out of trouble like Russia in the old days."

"Getting Ivankov is a major coup," said Stephen Handelman, author of the recent book "Comrade Citizen: Russia's New Mafia," "because it sends a message to Moscow that American law enforcement is finally getting serious about trying to stop the spread of organized crime here."

Russian gangs have been operating out of Brooklyn's Brighton Beach and Sheepshead Bay areas since the 1970s, when the Soviet Union allowed several hundred thousand Russians to emigrate to the United States. The criminal element that grew up around those immigrants mostly concentrated on gasoline tax scams and insurance fraud. But the collapse of the Soviet Union brought a new wave of Russian immigrants to New York City, by teaming up the small-time mobsters of the earlier era with highly organized gangs that have been operating in Russia for years.

Ivankov, law enforcement officials contend, was part of the new wave. In the old Soviet Union, he was the organized crime boss for the Russian far east, a post that, along with the Asiatic cast to his features, earned him the nickname "Yaponchik," or "Little Japanese." While in prison in the 1980s, he allegedly became one of the elite group of Russian crime bosses, commanding a huge network of thugs and underlings.

In late 1991 or 1992, he slipped into the United States on an illegal visa and joined his fellow emigres in the cafes and flashy nightclubs clustered under the elevated subway line in Brighton Beach.

"He was the most senior crime figure in the U.S., and basically he's been trying to take over the existing networks of small-time Russian groups and consolidate them into a professional organization," said Handelman.

In January 1993, the Russian Ministry of Interior notified the FBI of Ivankov's presence here. According to the complaint filed against Ivankov, mobsters under his direction extorted money from an investment advisory firm run by two Russian emigres, Alexander Volkov and Vladimir Voloshin.

The mobsters allegedly threatened to murder Voloshin's father in Moscow unless they were given millions of dollars in ransom. Voloshin's father was later killed -- after which Volkov and Voloshin were abducted and taken to a restaurant, where they agreed to pay $3.5 million in protection money, the complaint said. CAPTION: Vyacheslav Ivankov, in front, and Leonid Abelis, in white T-shirt, are escorted from FBI headquarters in New York.