Widebody; Low Wing; 2 Turbojet Eng
Flight Crew Air Transport Pilot (ATP)
Confusion; Distraction
Aircraft Equipment Problem Less Severe; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor; Ground Event / Encounter Other / Unknown
General Declared Emergency; General Flight Cancelled / Delayed; General Evacuated
Aircraft; Weather
Upon arrival at the aircraft; we noticed snow and ice on the plane. We coordinated for deicing in the gate. After all preflight procedures were accomplished; we started the Ground De/Anti-Ice checklist and indicated we were ready for de-ice. The ground crew cleared the aircraft of snow and ice. Maintenance called on the intercom to tell us de-ice was complete and to give us the information for the [holdover times]. The First Officer (FO) started his clock upon this notification. After 3 minutes; we continued the checklist. Soon after the APU Bleed was turned on; we heard the fire bell and received an ECAM warning of 'Cargo Comp Smoke'. We looked up to see it was the aft section. Within seconds we received multiple warnings from the other main deck  sections until all 3 were alarming. The smoke seemed to move forward. I asked the FO to open the door and take a quick look. We both saw the 9g net obscured by white smoke. He shut the door. I called ground control to declare the emergency and requested fire equipment; and then called for the Evacuation checklist. We accomplished the Evacuation checklist but did not discharge any agent since the smoke did not smell like smoke. The FO called out his window for the stairs; and we walked off the aircraft.
An air crew; after having been de-iced at the gate proceeded with their checklist and turned on the APU Bleed. A fire bell sounded followed by an ECAM warning of 'Cargo Comp Smoke'.  They declared an emergency and evacuated the airplane. The reporter concluded that 'De-icing fluid or water was in the ducting causing vapor which the aircraft sensed as smoke.'