Visibly nervous, the chairman of a local construction company asks that we lower our voices at the lunch table, and that his name be withheld from publication.

A few shady characters nearby are eavesdropping, he says. This neighborhood is the territory of one particularly violent faction of "yakuza," the powerful criminal underworld of Japan.

Every month, bargaining with the mid-level mobsters and shakedowns have become draining tasks.

"The yakuza have a hand in all sorts of industries, and working with them is just a part of doing business in this city," admits the executive, who himself was a mafia-connected negotiator for a construction company for almost 40 years.

But times are changing.

"We used to have a sort of harmony with these bosses," he laments. "They were enforcers, protectors who asked for our money to smooth out permits and deals, but who kept the battles to themselves. Now they're out of control."

Facing a shrinking pot of spoils, five mafia syndicates are waging an unusually vicious gang war in the coastal prefecture of Fukuoka. It's a rustbelt region on the southernmost island of Japan proper - known as Kyushu - which has the largest number of organized crime groups in the country, according to the government.

The fighting is sucking in police and, at times, innocents. Thugs have occasionally tossed hand grenades - known in yakuza parlance as "pineapples" - into their archenemies' headquarters, and into the homes of corporate executives who have declined extortionist requests from organized crime.

Last year, the Fukuoka Prefecture Police even became the first in Japan to offer bounties of $1,200 to citizens who reported suspects in possession of the explosives.

The yakuza, who number about 5,000 in the prefecture, have also shown they're willing to go after the officials who no longer tolerate their presence. The mayor of Kitakyushu and his family have received death threats, a motorbike gunman shot and wounded a retired detective, and gangsters gunned down the head of a construction company in front of his wife.

The attacks became more intense three years ago, when the local government declared war on the yakuza and passed a number of restrictions on them. The moves came alongside a growing body of national laws.

In late 2011, Japan passed the first laws completely outlawing payments to the yakuza. In Fukuoka, police have taken special measures to prevent members of certain gangs from gathering in groups of five or more in public; starting in June, they'll be banned from entering some business districts.

In December, public safety commissions in Kyushu took the additional step of identifying the Kudo-kai as the only syndicate among five that is "particularly dangerous," drawing on evidence that it is behind many of the grenade incidents. Two other organizations, the Dojin-kai and Kyushu-Seido-kai, were given a less restrictive ranking as "combative" that will be lifted in June.

It is a legal marker that gives law enforcement wider search and seizure powers. "This organized crime group, the Kudo-kai, is especially bad," explains Tetsuya Nishida, a police commander in the organized crime division. "They don't mind hurting civilians, and they go into construction companies and restaurants to get what they want."

The Kudo-kai did not respond to a request for an interview sent by snail mail to their headquarters.