US President Bush has called the worst blackout in North American history a "wake-up" call and said the US power grid must be upgraded.
President Bush is on a visit to California
Electricity supplies are gradually being restored - but recriminations have begun over which country caused the crisis.
The Americans and Canadians, whose electricity grids are inter-connected, are blaming each other.
The failure has affected some 50 million people in the US and Canada, causing blackouts in New York, Detroit, Cleveland, Toronto and Ottawa.
Millions of people are still without power and there is widespread disruption to public services especially transport.
New York's subways have started getting power back in a few locations but the lines remain closed. Once the electricity is restored it will take another six to eight hours to make sure the system is working properly.
Airlines have started flying again after hundreds of flights were cancelled. But delays have been reported, as airlines try to get through the back-log.
Many shops and restaurants in the affected areas stayed closed, and queues formed at petrol stations as people started running out of fuel.
By Friday afternoon, 75% of Toronto's power supply was back up, but the electricity company has warned it may have to impose rolling blackouts until the system can get back to normal.
The city's subway was not expected to be running until late Monday morning.
Many businesses in Toronto and Ottawa remained closed and a state of emergency was in force.
Workers stay at home
President Bush, visiting California, has praised his country's emergency response system, saying work since the 11 September 2001 attacks to upgrade procedures had paid off.
Now, the nation needed to make the same investment in its "antiquated" electricity grid, he said.
Many people stayed at home as requested rather than tackle the strained transport system.
The three biggest US car manufacturers remained closed on Friday, affecting tens of thousands of workers.
General Motors suspended work at 17 plants in three US states and Canada and hoped to get back to work on Monday. Ford closed all 23 of its facilities across the region, while Chrysler Group closed 14 of its plants.
In Cleveland the water supply was shut down because of the power failure.
In Detroit, officials urged people to boil water before drinking or cooking with it, because of low water pressure.
Some rural areas in Michigan were reconnected, but authorities warned that more populated urban centres would be powerless into the weekend.
In New York, parts of Manhattan, Brooklyn, Queens and the Bronx are now back on, but many others were without power.
With the subway closed, travel in the city has been reduced to a crawl, with packed buses, taxis and cars creeping along the streets.
New Yorkers were offered the following advice:
- Stay cool by drinking water, wearing light clothing and keeping the windows open
- Check on loved ones and the elderly
- Conserve energy in areas where power has been restored
- Some city swimming pools will be open as will cooling centres in each of New York's boroughs. Six hundred sprinklers will be on in the city's parks
- Check perishable foods in fridge
- Do not use the emergency number 911 for non-emergency calls, but 311 to get updates on the situation.
The exact cause of the blackout has not yet been pinpointed, but the US and Canada are blaming each other for the failure.
New York mayor Michael Bloomberg said the power cut began in Canada, but Ontario Premier Ernie Eves said it was triggered somewhere along Lake Erie in Ohio.
RECENT US BLACKOUTS
1996: 4m people hit by electricity outage across nine states
1977: lightning strike leaves New York without power for 25 hours
1965: power loss in north-east US and southern Canada hits 30m people
Mr Bloomberg said an event in Canada had put extreme demand on the New York power grid and the system collapsed.
"It appears... the load shedding that perhaps they should have done was not done in a way that prevented the New York power grid from having to try to supply power."