Yakuza gang members arrested for working too near a library due to strict rule

Four members of 's notorious Yakuza crime syndicate have been arrested on suspicion of conspiring to operate near a public library. They are part of the Sumiyoshi-kai union, which is part of the Yakuza criminal organisation, which is often likened to the Italian Mafia.

Planning laws in Japan dictate that Yakuza offices have to be outside a 200m radius of schools, libraries, other educational institutions, courts and museums. The arrests took place in , according to reports.

It is not illegal to run or be a member of a registered syndicate in Japan, but these are monitored by officials, including the country's National Police Agency.

There are currently 25 Shitei Boryokudan, which translates from Japanese into "particularly harmful groups", according to .

These are allowed to operate in the East Asian country as long as they don't commit crimes. The National Police Agency lists the addresses of some Yakuza organisations on its website.

Japan introduced a law in the 1990s to curb violent infighting between groups which had been spilling out onto streets, endangering the public.

Their increasing involvement in regular businesses such as real estate was also a growing matter of concern for the authorities at the time.

The Boryokudan Countermeasures Law, or Botaiho for short, was introduced in a bid to rein in their activities by conferring a certain level of legitimacy on them.

Due to Japan's constitution, Yakuza groups couldn't be banned because it would have violated members' right to freedom of assembly.

So they were tolerated but in ways that allowed the police to know where they were at all times, according to .

There are a number of theories surrounding the origin of the yakuza. Some believe they descend either from gangs of masterless samurai, known as rnin, who turned to banditry.

Others maintain they originate from bands of do-gooders who defended villages from the same samurai in the early 17th century.

Their lineage may also be traced back to bands of grifters and gamblers in Japan's feudal period, according to .

The word yakuza translates into "good for nothing". The term is believed to come from a worthless hand in a Japanese card game where the cards ya-ku-sa, meaning eight-nine-three, add up to give the worst possible total.

Police estimates put gang membership at a peak of some 184,000 in the 1960s, but by the early 21st century their numbers declined to about 80,000.