Police said they were searching for Justin Bourque, 24, and urged residents to stay inside and locked their doors.
"Three RCMP members have been killed," RCMP spokesman Paul Greene said in an emailed statement. "Two RCMP members were taken to hospital with what are believed to be non-life threatening injuries. The shooter remains at large."
Moncton, a city of about 70,000 people, is located in Canada's east coast province of New Brunswick. Mass shootings are relatively rare in Canada, which has stricter gun laws than the United States.
Local media published a photograph of a man in camouflage clothing and a black headband carrying a rifle. Reuters could not immediately authenticate the picture.
In an interview with CBC News, Greene said the officers were shot in the early evening after responding to reports of a man wearing camouflage clothing and carrying guns.
Regional healthcare outfit Horizon Health Network said on its Twitter feed that Moncton's hospital had received three patients with gunshot wounds.
The deaths of the three officers prompted an outpouring of grief on social media, with New Brunswick Premier David Alward and Federal Public Safety Minister Steven Blaney both saying they were shocked by the news.
The last mass killing of Canadian police took place in Mayerthorpe, Alberta in 2005, when a gunman killed four RCMP officers.
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