An Aerocontractors de Havilland Dash 8-400, flight NG-316 from Port Harcourt to Lagos (Nigeria) with 52 people on board, was enroute at FL240 about 75nm east of Lagos when the crew received a cargo smoke indication and the cabin started to fill with smoke. The crew declared emergency, performed smoke removal procedures and continued to Lagos for a safe landing about 18 minutes after the smoke was noticed. Nigeria's Civil Aviation Authority opened an investigation into the occurrence. The airline reported that about 75nm before Lagos cabin crew observed the cabin was misty and informed the flight crew. The flight crew made an announcement re-assuring the passengers. Shortly after the descent into Lagos had been initiated about 10 minutes later cabin crew observed a passenger entering the aft lavatory, the smoke detector activated shortly afterwards and the smoke in the cabin became more dense, the crew received an aft cargo smoke indication, too. The flight crew performed smoke removal procedures while cabin crew handed out wet towels. The aircraft landed safely about 18 minutes after the first observation of mist. It was determined that the smoke actually originated in the cargo section and was distributed into the cabin by the air conditioning system. An inspection of the aircraft did not reveal any anomaly.