But as the night wore on tempers began to fray. Two men from different yakuza gangs were playing. One was from an affiliate of the Yamaguchi-gumi, Japan's largest yakuza syndicate, which is based in Kobe in central Japan and has 23,000 members. The other was from the Kyokuto-kai, a 4,000-strong gang based in Tokyo. After midnight a dispute at the mah-jong table turned into an argument, which turned into a fight, and suddenly the Yamaguchi-gumi man was lying on the floor in a pool of blood, a samurai sword stuck in his stomach.
The police were quickly on the scene, and the Kyokuto-kai sword artist was arrested along with the owner of the mah-jong parlour. The death started vicious reprisals throughout Japan between the two gangs. Gunfire echoed even along streets in central Tokyo in the early hours of the morning, and police started to get worried.
Japan, in theory, is a gun-free society. In practice this is generally true. But the exceptions are the yakuza syndicates which run prostitution, gambling, loan- sharking and other seamy enterprises and who are all known to have arms. When a feud between two gangs breaks out, the normally safe streets of Japan can turn into a film set for shoot-outs between black-suited thugs with tinted-glass Mercedes and Uzi sub-machine-guns. Except the bullets are real.
Within hours of the mah-jong killing, a member of the Kyokuto-kai gang had been shot dead and three others injured in a reprisal. The Kyokuto-kai hit back, and the violence spread around the country. Bullets were fired into gang offices, cars were being shot up, and every morning the media would report on the tally of incidents from the night before.
Finally the violent conflict was ended in a very Japanese way - a third yakuza group, the Inagawa-kai, acted as mediators and both sides agreed to call off their battle-eager gorillas. The police, by contrast, seemed powerless. They threatened to close down the gangs' headquarters but did not even do that: apparently it would have taken a lot of time-consuming paperwork. Better to rely on the honour among thieves.
It came to be known as the 'six-day war'. At the end four people were dead, many more injured, and serious questions were being asked about the effectiveness of the new anti-yakuza laws which came into effect last year. Wasn't the new legislation meant to put an end to this kind of lawlessness?
The laws gave the police the power to close yakuza offices - why did they not do so? And should the laws not have been expanded to allow police to confiscate gangsters' illegally- obtained assets, as anti-racketeering laws in other countries permit? The police had no comments in the aftermath of the 'six-day war'.
Meanwhile the type of weekly magazines that include investigative reporting about mobsters along with their tales of pornographic actresses and politicians' secrets, probed deeper into the causes of the war. It has emerged that the mah-jong parlour killing was little more than a pretext for the show-down which had been brewing for some months over territory.
The Yamaguchi-gumi has been benefiting from the anti-yakuza laws to absorb smaller, more vulnerable gangs, thereby increasing its power throughout the country. But its main ambition is to penetrate Tokyo, since up to now the gang has been concentrated in the Kobe-Osaka area in central Japan. As they foraged through Tokyo's entertainment quarters of Shinjuku and Ikebukuro, they were increasingly stepping on the toes of the Kyokuto-kai. Both sides were spoiling for a fight.
According to the yakuza 'experts', the Yamaguchi-gumi's demonstration of its firepower during the 'six-day war' probably made it the winner. For the police, the most important thing was that the war had ended, and Japan could once again revert to being a safe, gun-free society. For the time being.