B737-300 FLT CREW ENCOUNTERS SEVERE TURB DURING CLB TO ALT. VMO EXCEEDED; ACFT PITCHED UP.

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ENCOUNTERED SEVERE TURB IN CLR AIR DURING CLB TO CRUISE ALT; BEGINNING ABOUT 14000 FT MSL AND CONTINUING THROUGH ABOUT FL180. I WAS PF. AUTOPLT/COMMAND STEERING ENGAGED; AT CLB THRUST AND FMS COMPUTED ECONOMY CLB SPD OF 330 KIAS. RIDE CONDITIONS WERE COMPLETELY SMOOTH PRIOR TO ENCOUNTER. ACFT 'BUNTED' OVER SLIGHTLY AND BEGAN TO ACCELERATE; FOLLOWED QUICKLY BY TURB ONSET (I INITIALLY THOUGHT I HAD ENCOUNTERED WAKE TURB; THEN THOUGHT IT WAS MACH BUFFET; AS THE CLB SPD WAS CLOSE TO VMO; AND WAS INCREASING DURING THE INITIAL PART OF THE ENCOUNTER). INTENSITY VERY QUICKLY INCREASED TO SEVERE CONDITIONS; WITH THE MACH WARNING TONE (CLACKER) SOUNDING; ALL IN RAPID SUCCESSION. AIRSPD EXCEEDED VMO BY ABOUT 10 KTS (ESTIMATE TOP SPD 325 KIAS). I DISENGAGED THE AUTOPLT; THEN DECREASED PWR AND BEGAN TO INCREASE PITCH TO BRING AIRSPD DOWN TO 280 KIAS (TURBULENT AIR PENETRATION SPD). I SELECTED PA ON THE COM PANEL AND DIRECTED THE FLT ATTENDANTS TO BE SEATED. ACFT CONTINUED TO PITCH AND ROLL SIGNIFICANTLY; WITH NOTABLE EXCURSIONS IN POSITIVE AND NEGATIVE G LOADS (ABOUT 0 TO 2.5). WE HAD BEEN CLRED TO CLB TO FL230; AND I WAS ABLE TO CTL THE CLB RATE AND GENERAL FLT PARAMETERS SHORTLY AFTER PASSING FL180. I CALLED FOR ALTIMETER SETTING 29.92 AND RE-ENGAGED THE AUTOPLT ABOUT FL210. TURB DIMINISHED TO 'LIGHT' BY FL200 AND CONTINUED THROUGH ABOUT FL250. I DETERMINED THE ACFT WAS FLYABLE AND RESPONDING NORMALLY; WITH NORMAL COCKPIT INDICATIONS. I THEN CALLED THE FLT ATTENDANTS ON THE SVC INTERPHONE TO CHK THEIR STATUS. THEY WERE OK; BUT THE 'B' FLT ATTENDANT TOLD ME HE HAD HIT HIS BACK ON SOMETHING AND IN SLIGHT PAIN; OTHERWISE WAS OK. THEY TOLD ME ALL PAX HAD BEEN SEATED/BUCKLED IN; AND NO DAMAGE OR INJURIES HAD OCCURRED. WE ADVISED ATC OF THE ENCOUNTER SHORTLY AFTER LEAVING FL180. THERE HAD BEEN NO MENTION MADE OF ANY ADVERSE RIDE CONDITIONS BY ATC; AND THEY SAID OUR RPT WAS THE FIRST ADVERSE RPT THEY HAD HEARD IN THE AREA.