OTTAWA (Reuters) - The Canadian province of Ontario on Sunday apologized for wrongly raising the alarm about an incident at a massive nuclear power station near Toronto and blamed a training exercise mistake.

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Angry local mayors demanded an inquiry, saying the emergency message about the ageing Pickering plant had caused unnecessary distress.

At around 7:30 a.m. ET (1230 GMT) cell phone users across Ontario - the most populous of Canada’s 10 provinces - received an alert about the supposed incident. Less than an hour later the Ontario Power Generation (OPG) authority said the message had been a mistake.

“The alert was issued in error to the public during a routine training exercise,” Ontario’s Solicitor-General Sylvia Jones said in a statement.

“The government of Ontario sincerely apologizes for raising public concern and has begun a full investigation to determine how this error happened.”

The initial message said there had been no abnormal leak of activity from the plant, which is located on the shores of Lake Ontario some 50 km (30 miles) east of downtown Toronto.

Pickering Mayor Dave Ryan said on Twitter that locals had been very troubled and added: “I have spoken to the province, and am demanding that a full investigation take place”.

Toronto Mayor John Tory complained that many of the city’s 3 million residents had been unnecessarily alarmed and also pushed for a probe, citing what he said were “far too many unanswered questions”.

Human error during a training exercise was also blamed for an incident in Hawaii in January 2018 when authorities issued a false alert about an impending ballistic missile attack.

OPG was not immediately available for comment.

The plant came online in 1971 and has a power-generating capacity of 3,100 megawatts when fully active. It is scheduled to be shut down in 2024.

“OPG has reminded everyone that they’re running an unneeded and aging nuclear station next to Toronto,” said Shawn-Patrick Stensil, a program director at Greenpeace Canada.

Some social media users posted images from the hit cartoon series the Simpsons, which features a nuclear power plant plagued by safety violations.

Cam Guthrie, the mayor of Guelph, a city west of Toronto, said “sending out a ‘hey there was an issue at a nuclear plant but we’re not going to tell you about it specifically and it’s not a big deal’ emergency text, is terrible”.