TOKYO (AFP) - A "one-eyed" gangster, the number-two man in Japan's biggest yakuza crime organisation, was convicted of extortion on Friday, winning a high-profile victory for the country's anti-mob police.

Kiyoshi "Mekkachi" Takayama, 65, who reputedly lost the use of his eye in a swordfight early in his career, was jailed for six years. His underworld nickname of "Mekkachi" means "one-eyed" in the dialect of western Japan.

Takayama is second in command of the Yamaguchi-gumi, a vast organised crime syndicate that had 27,700 members at the end of 2012, according to the National Police Agency.

He had been on 1.5 billion yen (S$19.9 million) bail for treatment of an illness since June last year. The details of his illness were not reported.

It was one of the highest bonds ever set by a Japanese court. The highest was 2 billion yen in a case of fraud by a meat importer.

Throughout his trial in the western city of Kyoto, the mobster had denied extorting 40 million yen from a local construction company with the help of an affiliated gang boss, reports have said.

Presiding judge Akihiro Ogura told the court: "The constructor's testimony that he had been the victim of extortion by the defendant is credible."

The judge added it was proven that Takayama "implicitly demanded protection money" through the local gang boss.

Like the Italian mafia or Chinese triads, the yakuza engages in activities ranging from gambling, drugs and prostitution to loan sharking, protection rackets, white-collar crime and business conducted through front companies.

The gangs, which are not illegal, have historically been tolerated by the authorities, although there are periodic clampdowns on some of their less savoury activities.