Japan's "yakuza" gangsters, traditionally preoccupied with local rackets, are covering their tattoos with pinstriped suits, branching out overseas and infiltrating legitimate businesses, police say.

Masahito Kanetaka, a National Police Agency specialist in organized crime, said in a recent interview that the centuries-old yakuza are becoming more difficult to control since they extended their operations to Hawaii, California and Southeast Asian capitals.Gang members are readily identified by their pointed white shoes and slicked-back hair. Many have missing pinky fingers, severed in a gang ritual of atonement for sins against the "Oyabun," or Godfather.

Yakuza also are noted for elaborate tattoos that often cover their entire bodies, from neck to wrists and ankles.

A Police Agency study in 1984 said the mob began making significant inroads into "intellectual" crime four years ago by posing as "Sokaiya," who buy small amounts of stock in companies and extort money by threatening to disrupt shareholders' meetings.

The study also said that more than half their estimated $6.2 billion annual income from drug trafficking.

"Before, yakuza were engaged in very specific activities - gambling, smuggling, bookmaking - with very limited contact with ordinary people," Mr. Kanetaka said. "Now they are expanding their net among the general public."

Police say stricter law enforcement and the consolidation of gangs has reduced yakuza numbers from a peak of 184,000 in 1963 to about 100,000 today.

The yakuza are back in the limelight after reports that a 43-year-old gangster caused the near-crash of an Osaka-bound Thai jetliner recently by accidentally setting off a smuggled hand grenade he was trying to hide in a cabin toilet.

The A-300 Airbus suffered sudden decompression and plunged five miles as its crew fought for control and finally made an emergency landing at Osaka.

Sixty-two of the 247 aboard were injured.

According to reports, the suspect, who was wounded in the incident and is under guard in a hospital, is a member of a group linked to the Osaka-based Yamaguchi-gumi, Japan's largest crime syndicate.

Police have refused to comment on the reports, saying only that the investigation into the near-disaster is continuing.

Mr. Kanetaka said the yakuza are becoming more violent.

"They have to fight harder to protect their turf," he said. "They need arms to threaten their rival groups, and to get into ordinary people's activities. They want to show their violence."

Police say at least two dozen mobsters have been slain in a gang war that broke out near Osaka in January 1985 between the Yamaguchi-gumi and the No. 2 Ichiwa-kai, whose members are estimated at 12,000 and 6,000, respectively.

Gang shoot-outs this year reached 218 in September, already twice the usual yearly total, Mr. Kanetaka said. Gun-running also is rising and authorities expect 1986 seizures to surpass last year's record haul of 1,787.

More than half of the smuggled arms captured last year came from the Philippines, he said. Press reports quote other officials as saying Manila- Osaka flights are often used for smuggling contraband.

The name yakuza first emerged in the 17th century feudal period and originally was an epithet. It is said to come from a card game, the object of which was to come as close as possible to 19. Drawing ya (eight), ku (nine) and za (three), totaling 20, is a worthless hand.