Paris Hilton -- Rips Radio Host ON THE AIR -- 'He's A F**king A**hole'

Paris Hilton went ballistic after a radio interview this morning ... launching into an expletive-filled rant against the DJ who had just interviewed her ... calling him a "f**king a**hole" ... without realizing she was STILL ON THE AIR.

Paris was on the phone with a morning team in Raleigh-Durham -- Bob and the Showgram -- and midway through the interview ... host Bob Dumas dropped the bomb: do you wish you had never made that sex tape?

Paris was annoyed ... but kept her cool ... blaming it on her XXX co-star Rick Salomon ... and calling him "a very sick man."

But Paris couldn't get off the phone fast enough ... and when she mistakenly thinks she's hung up and is off the air ... she goes off on the radio guy ... who's listening to the whole tirade.

Ya gotta hear his response ... it's pretty funny.