BELT LOADER SVCING E170 AT GATE CATCHES FIRE. FLT CREW RESPOND TO POSSIBILITY OF A CARGO COMPARTMENT FIRE. FIRE EXTINGUISHED WITH GND BASED EXTINGUISHERS.

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I WAS SEATED IN THE CAPT'S SEAT TALKING TO A NON 'REVING' FO WHEN I NOTICED SMOKE GOING BY THE FO'S WINDOW. I NOTICED THE SMOKE WAS GETTING THICKER BY THE SECOND SINCE THE FO I WAS TALKING TO WAS STANDING IN THE COCKPIT ENTRANCE I THOUGHT IT WOULD BE FASTER IF HE COULD LOOK OUT THE WINDOW TO SEE WHERE THE SMOKE WAS COMING FROM. MY FO WHOM I WAS FLYING WITH WAS GETTING FOOD; SO THE FO'S SEAT WAS EMPTY. THE NON 'REVING' FO LEANED OUT THE WINDOW AND SAID 'I THINK THE SMOKE IS COMING FROM THE PACK.' I TOLD HIM TO GO DOWNSTAIRS AND SEE WHAT WAS GOING ON. I THEN CLOSED THE L AND R PACK VALVES AND THE APU BLEED VALVE. I MOVED OVER TO THE FO'S SEAT TO LOOK BACK TOWARD FORWARD CARGO BAY BUT COULDN'T SEE ANYTHING BECAUSE THE SMOKE WAS SO THICK. I LOOKED FORWARD AND SAW A RAMPER RUNNING TOWARD THE AIRPLANE WITH THE DRY CHEMICAL EXTINGUISHER ON WHEELS. I'M NOW GUESSING THE SIDE OF THE AIRPLANE IS ON FIRE. JUST THEN THE FORWARD CARGO SMOKE WARNING ILLUMINATED. BASED ON THE INFO OF SEEING THE HVY SMOKE AND THE RAMPER RUNNING TOWARD THE ACFT; I ASSUMED RAMPERS WOULD BE OUT OF THE FORWARD CARGO BAY SO I PUSHED THE FORWARD CARGO SMOKE BUTTON. I TOLD THE FLT ATTENDANTS TO GET OUT OF THE ACFT; SHUT DOWN THE APU; TURNED OFF THE BATTERIES; AND EXITED THE ACFT. I IMMEDIATELY WENT DOWNSTAIRS TO SEE WHAT HAPPENED AND TO SEE IF EVERYBODY WAS ALRIGHT. I WAS TOLD THE BELT LOADER CAUGHT ON FIRE WHICH WAS PUT OUT BY DRY CHEMICAL RETARDANT. THE RETARDANT WAS EVERYWHERE INCLUDING THE SIDE OF THE AIRPLANE AND ENG COWLING. I ASKED THE RAMPERS IF THEY WERE ALRIGHT AND THEY SAID THEY WERE FINE. I LOOKED IN THE FORWARD CARGO BAY TO SEE IF ANYBODY WAS IN THERE. NO ONE WAS IN THE CARGO BAY. IT LOOKED LIKE NONE OF THE BAGS WERE TAKEN OUT. AFTER EVERYTHING WAS UNDER CTL ON THE RAMP; I WENT ON BOARD THE ACFT AND CALLED DISPATCH.