People throughout the Canadian province of Ontario awoke Sunday to an alarming alert of an "incident" at a nuclear plant just east of Toronto — only to later be told the message was a mistake.

The initial early morning emergency message popped up on the screens of cellphones throughout the nation's most populous province, saying an unspecified incident had occurred at the Pickering Nuclear Generating Station. It added there had been no abnormal release of radioactivity and said people didn't need to take protective action.

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More than an hour later, Ontario Power Generation later sent a message saying the alert "was sent in error. There is no danger to the public or environment. No further action is required." The notification message also was pushed onto to television screens.

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Pickering Mayor Dave Ryan said he's demanding a full investigation into the error.

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

An emergency alert issued by the Canadian province of Ontario reporting an unspecified 'incident' at a nuclear plant is shown on a smartphone. AP Photo / Robert Gillies
An emergency alert issued by the Canadian province of Ontario reporting an unspecified 'incident' at a nuclear plant is shown on a smartphone. AP Photo / Robert Gillies

Toronto Mayor John Tory joined him, tweeting that "there are far too many unanswered questions" and noting it went out across the province of 14 million people.

"Last night may have been the wrong night to start watching Chernobyl," tweeted CBC journalist David Common.

"It's comforting to know that when the apocalypse does come, we'll all still be snarking on Twitter," Emmett Macfarlane, a University of Waterloo professor, tweeted.

The US Department of Homeland Security's inspector general recommended changes to emergency alert system in the United States after Hawaii officials in 2018 mistakenly warned the public about a nonexistent incoming ballistic missile. An Hawaii Emergency Management Agency employee sent the missile alert to cellphones and broadcasters, triggering panic until the agency sent another message 38 minutes later notifying people it was a false alarm.

Pickering, opened in 1971, and had been scheduled to be decommissioned this year, but the province's government committed to keeping it open until 2024. Decommissioning is now set to start in 2028.

A jogger runs along the beach past the Pickering Nuclear Generating Station, in Pickering, Ontario. Photo / Frank Gunn, The Canadian Press via AP
A jogger runs along the beach past the Pickering Nuclear Generating Station, in Pickering, Ontario. Photo / Frank Gunn, The Canadian Press via AP

It generates 14 per cent of Ontario's electricity, and is responsible for 4500 jobs across the region, according to Ontario Power.

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It has experienced several earlier incidents. In 2011, a pump seal failure caused the spill of more than 19,200 gallons (73,000 litres) of demineralised water into Lake Ontario, though with no significant risks to public health, according to local authorities.

In 1994, the plant automatically shut down after a faulty valve caused 132 tons of heavy water to spill. It was the first time a Canadian nuclear reactor had to use the emergency core cooling system to prevent fuel overheating.