Firefighters and paramedics were called to Haymarket at 7.40pm after the intense explosion rocked a food court on the corner of Little Hay Street and Sussex Street.

The force of the blast was felt at the building's loading dock and required the building to be evacuated, Superintendent Paul Johnstone from Fire and Rescue NSW said.

"The explosion blew out the roller shutter door, injuring people in the immediate vicinity," Mr Johnstone said.

"There was no fire spread after the explosion. The sprinklers activated and contained any fire."

The blast injured at least 16 people, who were assessed by paramedics for cuts and lacerations.

Seven were taken to hospital.

One adult and a young boy, believed to be two years old, were taken to Sydney Children's Hospital at Randwick in the same ambulance.

The boy had a laceration to his head and lip, a spokesman for NSW Ambulance said.

Three adults were taken to Royal Prince Alfred Hospital, and two were taken to St Vincent's Hospital.

Exact injuries of the adult patients were not known, but it is understood they suffered various cuts and lacerations.

Two hundred people who were inside the five-storey building at the time of the blast were not injured, but were evacuated as a precaution in case gas remained in the building.

Police cordoned off the footpath as firefighters investigated.

There was no damage to surrounding buildings, and the blast is not believed to be suspicious.

Eight fire trucks and 40 firefighters remained on the scene for hours after the explosion, as utility workers were called to isolate the building's gas.

The entire building would be checked for pockets of gas before residents were allowed back in, Mr Johnstone said.

Firefighters do not believe the explosion affected the building's structural integrity.

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