More than 100 prisoners staged a riot Sunday morning at the central prison in Yala, one of the insurgency-plagued southernmost provinces of Thailand.The rioters burnt down an office of police guards in the jail, which holds about 600 prisoners and were engaged in fistfights with the jailers.The motive of the riot is not yet clear, while the governor of Yala province is conducting a negotiation with the rioters.