A loose-knit Asian Mafia with criminal operations in Hong Kong, Taiwan and more than a dozen North American cities is responsible for a fifth of the heroin imported into the United States as well as for extensive gambling, extortion and protection rackets on the East and West coasts, according to the President's Commission on Organized Crime.

Testifying today at the commission's first hearings since it was given subpoena and immunity-granting powers by Congress, Attorney General William French Smith said the "Japanese Mafia," known as the Yakuza, and Chinese Triad Societies are part of "new crime cartels emerging in the Far East and spreading to the West."

"We must stay in front of the emerging crime groups so they do not become so entrenched that they become all but impossible to root out, as indeed happened in earlier years with the Mafia," he said.

U.S. Appeals Court Judge Irving Kaufman, who heads the commission charged with investigating and publicizing organized crime, said the Asian groups are "sophisticated organized criminal groups," which have "formed ties to La Cosa Nostra and control international narcotics networks trafficking in heroin from Asia's Golden Triangle." They also have extensive money-laundering operations, he said.

"Many of these operations are highly structured and disciplined and have been operating virtually unnoticed by the American public," Kaufman said.

James D. Harmon Jr., the commission's executive director, said the three-day hearings also will examine violent Vietnamese gangs operating in southern California, Houston, New Orleans and Arlington, Va.

FBI organized crime section chief Sean McWeeney was quoted earlier this year as saying that the FBI was deciding whether to target Asian organized crime as one of its nationwide enforcement priorities.

Sgt. Barry Hill, head of a Toronto police unit that deals exclusively with Chinese organized crime, said that Chinese Triads, 300-year-old secret societies that operate extensively in Hong Kong and Taiwan, are well-entrenched in Toronto, with at least 300 members.

He said a Toronto leader accompanied Chinese gang leaders from San Francisco and Los Angeles in January 1983 to a meeting in Hong Kong with the leader of a Hong Kong triad. The reputed head of Boston's Chinese triad was to accompany the group, but did not for an unspecified reason, he said.

The four North Americans have been subpoenaed. Two were jailed for refusing to testify, although one later relented. One has fled and one is negotiating with the committee, according to Harmon.

Testimony from law enforcement officials from Chicago, San Francisco, Oakland, Calif., and New York described violent gangs operating in those cities' Chinatowns, but they had few details about alleged connections between the Asia-based Triads, which include non-criminal elements and the U.S. Tongs, Chinese social organizations that include legitimate businessmen.

John Feehan, a federal drug enforcement administration agent here, testified that two major gangs -- the Ghost Shadows and the Flying Dragons -- have this year extended their Chinatown operations from extortion and loan sharking to heroin and cocaine trafficking. He said, however, no connection between the gangs and Hong Kong Triads has been established.

According to a map displayed by the commission, Tongs that include criminal elements are active in Toronto, Boston, New York, San Francisco, Los Angeles, Atlanta, Dallas, Chicago, Kansas City, Omaha, Minneapolis, Detroit, Cleveland, Pittsburgh, Columbus, Ohio, Charlotte, N.C., Jacksonville, New Orleans, Miami, Houston, San Antonio, Providence, R.I., Hartford, Conn., Albany, N.Y., Baltimore, Richmond and Norfolk.

In the District of Columbia, according to the map, the On Leong and Hip Sing Tongs are active.

Law enforcement officials and two hooded witnesses who testified from behind screens said gangs from different cities cooperate by doing contract murders for each other and hiding each other's criminals from police.

The 19-member commission is due to report its findings in March 1986 and make legislative and policy recommendations.