The building, located in the city's eastern Ange Raphael neighbourhood, caved in around midnight.
"Twelve died and the others are in hospital for medical attention," the governor of the surrounding Littoral region told journalists at the site, saying 31 people in total were affected.
The situation was under control and rescue teams were making sure no one remained under the rubble, he said.
Though the cause of the collapse was unclear, local residents said it looked deteriorated with exposed and worn out rods.
Police said that At least twelve people were killed and nearly three dozen injured in a building collapse, Cameroonian authorities said on Sunday.
"The casualty figures may be higher. Rescue workers, assisted by Cameroon government troops, are still digging the wreckage to see if more bodies can be recovered," said Samuel Dieudonne Ivaha Diboua, governor of Cameroon's Littoral region.
The military's fire brigade has been ordered to join the country's Red Cross and other rescue services in searching for survivors trapped under the rubble.
Residents living in the Ndogbon neighborhood where the incident took place said they are in shock.
"We heard people screaming ... and struggled to help some out of the wreckage, but could not do it with our spades and (garden) hoes," said Gaspard Ndoppo, who lives near the collapsed buildings.
Building collapses happen often in Douala, sometimes due to natural disasters such as landslides and other times because of poor construction, locals say.
Douala's city council is currently demolishing houses in high-risk zones susceptible to floods or landslides.
The building that collapsed on Sunday was not marked for demolition.