"We can confirm that a party has been shot, the extent of the injuries are not yet known. Emergency run being done to Toronto," Peel Regional Police said in a Twitter message.
A police spokeswoman told public broadcaster CBC that an emergency call was received at 10:59 am after "shots were fired within the courthouse."
The officer said the packed building in Brampton, Ontario, was immediately locked down, and the injured person was rushed to hospital.
"We believe a few others were also injured," she added.
Canadian television showed images from a news helicopter of an ambulance escorted by several police cars racing down a highway toward a local hospital, and dozens of police cars surrounding the courthouse.