Storms caused widespread power outages in parts of the upper Midwest and Northeast, and in southern Ontario in Canada, as the region baked in a heat wave late Friday, the authorities and power companies said.

About 1 million customers were without electricity in a band stretching from Illinois to New York and into Ontario on Friday night, according to PowerOutage.com, a tracking site. The authorities blamed down power lines for many of the outages.

The combination of extreme heat and power outages prompted urgent safety warnings. In New York City, Mayor Zohran Mamdani said on social media that outages could be dangerous even at night, suggesting that residents without power go to overnight cooling centers provided by the city.

Con Edison, the utility that provides power to most of New York City, said in a statement that thunderstorms had knocked out power to thousands of customers across the five boroughs and in Westchester County, exacerbating the strain on the grid caused by the prolonged heat.

Governor Mikie Sherrill of New Jersey said on social media that severe storms had knocked out electricity across the state’s northern and central counties, and urged residents to avoid downed power lines. The storms also disrupted several train lines, NJ Transit said.

The police in Warren, a township in New Jersey about 30 miles west of New York City, said that the outage could be long. “Utility crews have been notified but power restoration is expected to take an extended period of time due to the widespread damage,” they said in an alert.

The small borough of Alburtis in eastern Pennsylvania declared a state of emergency after significant wind damage, including downed trees and power lines. Crews were working to restore power in the area, the local fire department said on social media.

Michigan’s two largest utilities, DTE Energy and Consumers Energy, sent out crews on Friday night to tackle widespread outages. Power was expected to be restored by the end of Monday for 95 percent of the affected customers, DTE Energy said.

In northern Ohio, the Hambden and Chardon fire departments responded to a surge of emergency calls regarding downed power lines and trees.

Severe storms caused widespread outages in Tennessee, too, the utility EPB said on its website.

The storms also knocked out power for thousands of customers in the Canadian province of Ontario, the electricity company Essex Powerlines said on social media. Hydro One, another utility in Ontario, said that crews were still working on Friday to deal with “extensive damage” caused by thunderstorms earlier in the week.