A DHC8-100s air-conditioning pack seized in flight causing smoke in the cabin. An emergency was declared with a diversion to a nearby airport where the passengers were deplaned on a taxiway. The smoke's cause was determined after the fact.

Narrative

I was operating flight from ZZZ-ZZZ1. Approximately 20 minutes into the flight; my First Officer and I noticed an unusual odor. As we were discussing it; our Flight Attendant called and said she smelled smoke in the cabin. As I was telling her we smelled something too; we noticed intermittent smoke coming from the cockpit eyeball vents. Our Flight Attendant then told us that she now saw smoke also; coming from the overhead gasper vents in the cabin. We completed the appropriate checklist; and as soon as the engine bleeds were selected off; the smoke stopped. At the time; we were approximately 20 miles from the ZZZ VOR; and talking to ZZZ Approach Control. We elected to declare an emergency and divert to ZZZ. I briefed the Flight Attendant; the passengers; and asked Approach Control to call our company and inform them that we were diverting to ZZZ. After a normal landing; we cleared the runway and shut down on Taxiway B. Crash Fire Rescue met the aircraft immediately and performed an exterior inspection with no damage noted. There was still some residual smoke in the cabin; so we agreed to deplane the passengers while on the taxiway. The passengers were then taken inside the terminal via airport vehicles to be re-accommodated. The aircraft was towed to remote parking where local contract maintenance determined that the air conditioning pack (air cycle machine) had seized. No injuries or medical issues were reported by any passenger or crew member.