Authorities in nine states worked to restore basic services such as public transit and electricity.
In New York City, nearly 500,000 customers were without power. In Manhattan, many of the 220,000 customers without electricity were south of Midtown's 34th Street. Parts of Queens and Staten Island also had no electricity Thursday. "Restoring power will take a lot of time," the mayor said.
Gov. Andrew Cuomo, in a letter to utilities, warned of consequences if authorities discover that they failed to prepare properly. "Under such circumstances, I would direct the Public Service Commission to commence a proceeding to revoke your certificates," he wrote.
That message was not lost on its intended targets. "We're doing our damnedest to get our power back as quickly as possible," said John Miksad, senior vice president of electric operations at Con Ed.
New York's vast transit network remains hobbled. The Metropolitan Transit Authority said 14 of the city's 23 subway lines were running and a flotilla of 4,000 buses was attempting to take up the slack. For some, Thursday's commute into Manhattan from the outer boroughs took five hours.
Bloomberg predicted that would ease as tunnels are cleared of water, power is restored to subway lines and ferries resume service.
Getting water out of the tunnels is "one of the main orders of business right now," Cuomo said.
Broadway theaters reopened Thursday, and organizers vowed to hold the New York City Marathon as scheduled on Sunday. Event organizer Mary Wittenberg said the race wouldn't divert resources from the recovery.
Three days after Sandy barreled ashore in southern New Jersey, search-and-rescue crews were going door-to-door in some neighborhoods looking for people, particularly the elderly, who may have been stranded by the power outages, the debris and remaining floodwater.
Sandy killed at least six people in New Jersey, said Gov. Chris Christie, who had warned people in low-lying areas to evacuate.
Christie asked for patience as crews worked to restore electricity to more than 2 million power company customers.
The federal government shipped 1 million meals Thursday to New York, where National Guard troops were distributing them to people in need, Cuomo told reporters.
The storm dumped up to 3 feet of snow in West Virginia and Maryland, leaving thousands without power.
Nearly 3.5 million customers across the eastern United States were still in the dark Thursday, down from nearly 8 million in its immediate aftermath.
By Thursday, Sandy's remnants had headed into Canada.
The National Weather Service predicted a nor'easter next week from the mid-Atlantic states into New England. But the forecast said the storm would be far weaker than Sandy.