"I could see the look on the first paramedic, saying 'Oh my God,"' Toronto emergency medical services Commander David Ralph said with a laugh.
The victims' zombie costumes made it difficult at first for crews to assess the severity of their injuries, Toronto emergency medical spokesman Peter Macintyre said. Officials said none of the injuries were considered life-threatening.
Paramedics responded to the call from Cinespace Film Studios around 8:30 a.m., and Toronto Police Sgt. Andrew Gibson said responders quickly figured out which zombies were injured and which were just in character. "It did kind of catch us off guard when we walked in," he said.
The victims had suffered leg, back and arm injuries. Twelve were taken to a hospital.
Police Constable Tony Vella said the actors had been filming a scene on a high wheeled platform. The platform moved as they were going to another platform, leaving a gap into which the zombies fell.
The film's producers, Davis Films/Impact Pictures (RE5) Inc., Constantin Film International GmbH, issued a statement that said the accident involved background actors. They said a 4-foot (1.2-meter) high platform separated from the set, creating a gap.
They said one actor suffered a cracked rib, one a broken leg and one a possible back injury.
Seven of the injured have already returned to the set, they said.
"Resident Evil: Retribution" is the fifth installment of a franchise based on the popular video game series and will be distributed by Screen Gems and Sony Pictures.
Lead actress Milla Jovovich was not on set at the time of the accident, according to her representative Christine Tripicchio.
Vella said the incident is classified as an industrial accident. Ontario's Ministry of Labor is investigating.