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The yakuza emerged from misfit pedlars and gamblers in the Edo period (between 1603 and 1868) that formed into criminal gangs. During Japan’s turbo-charged modernisation, they reached deep into the economy; after the second world war they grew powerful in black markets. Their might peaked in the 1960s with an estimated membership of 184,000. At their zenith, they had strong links to conservative politicians and were used by the Liberal Democratic Party (LDP), Japan’s post-war political behemoth, to break up unions and left-wing demonstrations. Such ties may not have completely faded.
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Interesting factoid, the Yakuza are not illegal, they even have offices and carry business cards.

http://extragoodshit.phlap.net/index.php/why-the-yakuza-are-not-illegal/