The injured have been taken to hospital. All were awake and able to speak, according to SOS Alarm.
The motorcycle rally on Saturday was a big event in Svartå in Närke, near the border with Värmland, with many people traveling there.
It is a motor event where 200 to 300 people have gathered, says Anders Dahlman, press spokesperson for the police in the Bergslagen region.
The meeting came to an abrupt end when the explosion occurred, according to police.
A representative for the organizer, the Swedish Transport History Society, told SVT that the accident occurred when a Rolls-Royce engine was being demonstrated. Parts of the engine are said to have been scattered about ten meters from the scene of the accident.
"When it hit, it hit hard. If this hits you in the head, then it's goodnight," says Fredrik Johansson, who is holding a cast iron part from the engine that flew away in the explosion, to P4 Värmland.
The rescue operation ended shortly after lunch.
The police have launched a preliminary investigation into the incident.
