INTERNATIONAL B777-200 DEVELOPED SMOKE AND FUMES IN THE CABIN. THE SOURCE COULD NOT BE IDENTIFIED. THE FLT CREW DECLARED AN EMER AND DIVERTED.

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FLT ATTENDANT ADVISED ME THAT THEY HAD A STRONG ELECTRICAL SMELL AT DOOR 3R AND IT WAS GETTING STRONGER IN THE BACK OF THE ACFT. I ADVISED THE PURSER TO SHUT OFF THE IFE AND PWR PORT SWITCHES. I ALSO ADVISED THE FLT ATTENDANTS TO CHK THE COMPUTERS IN THE AREA. I ALSO STARTED TO RUN THE SMOKE/FUMES ELECTRICAL CHKLIST AND SHUT OFF THE GASPER FAN AND THE RECIRCULATION FANS. I ALSO GOT THE RELIEF PLT UP FROM BREAK AND HAD HIM GO BACK AND SEE WHAT WE HAD. HE CALLED AND SAID THAT THE SMELL WAS GOING AWAY. AT THE SAME TIME; THE FO WAS GETTING THE LATEST WX FOR POSSIBLE DIVERSION. A COUPLE OF MINS LATER THE RELIEF PLT CALLED AND SAID THAT ALL TRACES OF THE SMELL WERE GONE. I ELECTED TO CONTINUE TO OUR DEST. ABOUT 3.5 HRS LATER; N OF ZZZZ; DURING THE SECOND FOOD SVC; THE PURSER AWOKE ME AND TOLD ME THAT THE SMELL HAD RETURNED AND THAT THEY COULD NOW SMELL IT IN FIRST CLASS. BEFORE RETURNING TO THE FLT DECK; I HAD THE PURSER TURN ALL GALLEYS OFF AND HAVE THE SHADES OPEN AND ALL LIGHTING TURNED OFF. ON ENTERING THE COCKPIT; I COULD SMELL THE FUMES ALSO. AFTER I GOT IN THE SEAT; I DECLARED A MAYDAY AND TURNED TOWARD ZZZZ1. COMPLETED CHKLISTS AND BRIEFED THE PURSER FOR OUR ARR. THE RELIEF PLT COMMUNICATED WITH DISPATCH WHILE I WORKED WITH ATC AND SET UP THE APCH AND BRIEFED THE APCH UTILIZING THE PLATE. COMMENCED THE APCH; AT 10000 FT TURNED PACKS OFF IN CASE OF EVAC. FO MADE AN UNEVENTFUL LNDG. AFTER LNDG; HAD CFR CHK ACFT AND CARGO HOLDS FOR SMOKE OR FIRE. ALL WAS CLR AND TAXIED TO GATE. AT GATE; FIRE CHKED CARGO AND CABIN FOR FIRE AND SMOKE USING IR DEVICE AND NOTHING FOUND. ON APCH; SMELL WENT AWAY. I WOULD LIKE TO COMMEND THE CREW FOR THEIR PROFESSIONALISM IN THIS SITUATION. THE FO HANDLED ALL THE FLYING AND SITUATIONAL AWARENESS. THE RELIEF PLT HANDLED A LOT OF THE INTERNAL COMS AS WELL AS THE COMS WITH DISPATCH AND ZZZZ1 OPS. THE PURSER HANDLED ALL FLT ATTENDANT COMS AND WAS READY FOR EVAC OR FIRE IF NEEDED. THE ONE PROB NOTED WAS THAT WHEN THE FLT ATTENDANTS READIES A PBE IT WAS VERY DIFFICULT TO OPEN AND THEY WERE AFRAID THEY WOULD TEAR THE PBE ITSELF. THEY STATED THAT THEY WERE NEVER SHOWN HOW TO OPEN THE PACKAGE JUST HOW TO PUT IT ON.