B757-200 CREW HAD SMOKE IN THE COCKPIT IN ZOB CLASS A.

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UPON REACHING 31000 FT; SMELLED SMOKE. CONTACTED LEAD FLT ATTENDANT TO ASK IF THERE WAS SMOKE IN CABIN. ODOR SUBSIDED IN COCKPIT. MINS LATER SMOKE WAS POURING OUT OF CAPT'S SIDE PANEL AND FLOOR. BOTH PLTS DONN OXYGEN MASKS AND SMOKE GOGGLES. DECLARED EMER. REQUESTED DIVERSION TO CLE. FLT ATTENDANT NOTIFIED OF DIVERSION AND COMPANY NOTIFIED. TOLD ATC FUEL AND NUMBER OF PAX ONBOARD. AT THE SAME TIME ACCOMPLISHED IMMEDIATE ACTION ITEMS ON QRH; THEN CONTINUED QRH PROCS. SMOKE CONTINUED UNTIL LNDG. SAFE LNDG. SMOKE SUBSIDED ON ROLLOUT. CLRED RWY. NO FURTHER SMOKE OR FIRE INDICATED. ADVISED PAX TO REMAIN SEATED. TAXIED TO GATE AND DEPLANED PAX. NO CREW OR PAX INJURIES. SUPPLEMENTAL INFO FROM ACN 538507: UPON REACHING 31000 FT; THE CAPT SMELLED SMOKE; CONTACTED LEAD FLT ATTENDANT TO ASK IF THEY SMELLED SMOKE OR SEE ANYTHING IN CABIN. ODOR SUBSIDED; A FEW MINS LATER CAPT INDICATED/OBSERVED SMOKE POURING FROM AREA BY HIS L KNEE. WE BOTH DONNED SMOKE GOGGLES AND OXYGEN MASKS. WITH DIRECTION FROM CAPT; DECLARED EMER/MAYDAY AND DIVERTED TO CLE. FLT ATTENDANTS NOTIFIED OF TIME REMAINING; NATURE OF EMER; INSTRUCTIONS; ETC; COMPANY AND OPS IN CLE NOTIFIED. ATC ADVISED OF NUMBER OF PAX AND FUEL ONBOARD. WHILE THIS WAS GOING ON; WE ACCOMPLISHED EMER CHKLISTS AND COMPLETED QRH ITEMS. SMOKE CONTINUED UNTIL LNDG. SAFE LNDG AND SMOKE SUBSIDED ON ROLLOUT; CLRED RWY AND AFTER FURTHER OBSERVATIONS BY CREW AND FIRE PERSONNEL; TAXIED AND DEPLANED PAX AT GATE. NO CREW OR PAX INJURED. POSSIBLE CAUSE: OLDER WIRING IN AN OLDER B757 THAN MOST.