A B757-200 ENCOUNTERED SEVERE TURB AT CRUISE ALT WITH SEVERAL FLT ATTENDANT AND PAX INJURIES; BUT NO ACFT DAMAGE.

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WE WERE CRUISING IN CLR; SMOOTH AIR WITH THE SEAT BELT SIGN TURNED OFF. WE WERE APCHING SOME CLOUDS AT ALT AND TURNED ON THE WX RADAR TO SEE IF ANY SIGNIFICANT WX WAS BEING OBSCURED BY THE CLOUD DECK. THERE WAS NO CUMULO NIMBUS ACTIVITY OR ANY OTHER HAZARDOUS WX BEING DEPICTED ON THE RADAR. AS A PRECAUTION I ELECTED TO TURN ON THE SEAT BELT SIGN. AFTER CONCURRING WITH THE FO I DECIDED TO HAVE THE FLT ATTENDANTS TAKE THEIR SEATS. I CALLED THE PURSER AND TOLD HER TO HAVE EVERYONE BE SEATED. I DID NOT MAKE A PA ANNOUNCEMENT; BUT RELIED ON HER TO PASS THE WORD. WITHIN ABOUT A MIN WE ENTERED THE CLOUDS. WITHIN A FEW SECONDS WE ENCOUNTERED SOME LIGHT TURB. AFTER A FEW MORE SECONDS WE EXPERIENCED A VIOLENT JOLT WHICH LASTED ABOUT 5 SECONDS. AFTER ABOUT 30 MORE SECONDS WE EXITED THE CLOUDS INTO SMOOTH AIR. UNFORTUNATELY; SOME FLT ATTENDANTS RPTED THAT THEY WERE NOT IN THEIR SEATS AND EXPERIENCED SOME INJURIES. ONE WAS TRYING TO GET A CHILD PAX OUT OF THE LAVATORY. EVIDENTLY SOME PAX WERE NOT IN THEIR SEATS IN SPITE OF THE SEAT BELT SIGN BEING ON. THE ACFT DID NOT SUSTAIN DAMAGE. WHEN WE LANDED; I HAD PARAMEDICS MEET THE ACFT AT THE GATE. 3 FLT ATTENDANTS AND 2 PAX INDICATED THAT THEY WANTED TO BE CHKED OUT FOR INJURIES ALTHOUGH NO ONE COMPLAINED OF ANYTHING OTHER THAN POSSIBLE STRAINS OR PAINS. THERE WERE NO INDICATIONS OF BROKEN BONES AND NOBODY IMPACTED THE CEILING OF THE PLANE. THERE WERE NO CUTS OR VISIBLE BRUISES. A TURB RPT WAS COMMUNICATED TO ATC THROUGH OUR DISPATCHER WHOM WE IMMEDIATELY NOTIFIED.