The emergency alert went out shortly before 7:30 am (1230 GMT). Though intended for residents living within a 10-kilometer (six-mile) radius of the Pickering Nuclear Generating Station, it went to all residents in Ontario province.

About an hour later, the Ontario Power Generation company that manages the nuclear plant announced on Twitter that the alert was issued by mistake.

"There is NO active nuclear situation taking place at the Pickering Nuclear Generating Station," it said.

"The previous alert was issued in error. There is no danger to the public or environment."

The Ontario provincial government apologized, saying the alert was issued during a routine training exercise.

"The Government of Ontario sincerely apologizes for raising public concern and has begun a full investigation to determine how this error happened," Solicitor General Sylvia Jones said.

One of the world's largest nuclear power plants, the Pickering facility is located about 50 kilometers east of Toronto, Canada's most populous city, with three million inhabitants.

Several local officials, including Pickering mayor Dave Ryan, immediately demanded an explanation of the incident.

"Like many of you, I was very troubled to have received that emergency alert this morning," Ryan said on Twitter.

"While I am relieved that there was no actual emergency, I am upset that an error such as this occurred. I have spoken to the province and am demanding that a full investigation take place," he said.

Toronto mayor John Tory also called for a probe, saying area residents had been "unnecessarily alarmed by this event."