'Pawn Stars' Rick Harrison -- Violent Biker Gang Tricked The Old Man

The "Pawn Star" patriarch known as The Old Man was unwittingly lured into helping a violent motorcycle gang beat serious criminal charges ... so claims The Old Man's son.

The Old Man recently hosted a Valentine's Day party for the infamous Vagos biker gang at his Vegas home. The gang wanted The Old Man to host because of his fame, banking on the fact that lots of people would attend and plunk money down to win various raffles.

Now here's the alleged scam. Sources say the proceeds from the raffle went right into a slush fund used to pay for lawyers for members of the gang that commit crimes. And the Vagos crew gets into LOTS of trouble.

A 4-year federal investigation resulted in the arrests of 32 gang members -- mostly Vagos -- for crimes ranging from gun and drug trafficking to robbery.

Rick Harrison, "Pawn Stars" star and also son of The Old Man, tells TMZ the gang took advantage of his 72-year-old dad who had no idea he was fronting a party for nefarious purposes.

Rick also knows exactly how is father got bamboozled. Rick's brother, Joseph, is a long-time member of Vagos with a big-time rap sheet.