The initial emergency message sent to cellphones of residents in the region said the incident had occurred at the Pickering Nuclear Generating Station, though it added there had been no abnormal release of radioactivity from the station and said people didn't need to take protective action.

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 text message also was pushed onto to television screens.

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.

Toronto Mayor John Tory joined him, tweeting that "there are far too many unanswered questions."

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.

It generates 14 per cent of Ontario's electricity, and is responsible for 4,500 jobs across the region, according to Ontario Power.

It has experienced several earlier incidents. In 2011, a pump seal failure caused the spill of more than 19,200 gallons (73,000 liters) of demineralized 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,