EMB-170 Captain reported a GPU arc fire during deplaning. Captain expedited passenger deplaning as ramp and fire personnel responded.

Narrative

As we were in the process of deplaning; the ramp crew attached the ground power to the aircraft. I saw that ground power was indicating available and selected the GPU button on the overhead panel; then switched the APU off for a normal shutdown sequence. At that time; some of the overhead and sidewall lights caught my attention as they were flickering on and off; seemingly at random. I attributed this to the electrical transfer to ground power; but assumed it was a minor fault; and attempted to reset the sidewall lights at the FWD FA (Flight Attendant) panel. I returned to the flight deck; and shortly after that a loud hissing noise similar to radio static was heard as the aircraft lost electrical power. The FWD FA brought to my attention that the GPU receptacle was arcing; and I promptly shut BATT 1 and BATT 2 off in response. We shouted to the remaining passengers to get off quickly through the L1 door; shortly after all passengers deplaned we left as well. By this time most of the arcing had stopped; and the ramp personnel had mounted a response with the wheeled fire extinguisher. I momentarily returned to the flight deck to retrieve items; and a significant amount of electrical smoke had filled the flight deck. After the fire department arrived we opened the rear doors and flight deck windows to ventilate the aircraft. No further arcing or fire was observed.