County's first away win of the campaign was followed by a mass touchline brawl involving players, substitutes and officials from both clubs immediately after the final whistle.
Ben Davies seized on an error from Chris Whelpdale to slam the opener through the legs of Posh keeper Joe Lewis from close range and he doubled the lead three minutes later after being released by Neal Bishop.
Posh needed only two minutes to reduce their arrears when Gaby Zakuani rose to head home a deep Grant McCann corner but Craig Westcarr grabbed a third County goal out of nothing two minutes before the break, beating Lewis with a bouncing 20-yard drive into the far corner.
Tommy Rowe slammed a dipping long-range half-volley against the bar before Craig Mackail-Smith prodded in from an acute angle in the 59th minute.
But McCann was dismissed eight minutes later for a crunching foul on Alan Judge, while Ricky Ravenhill received his marching orders in stoppage time for kicking the ball away.