Utility workers fanned out throughout the East Coast to fix downed electrical lines and restore power to millions of people left in the dark by Hurricane Irene.

Power is returning to thousands of customers in the southern coastal states, the first area hit by Irene on Saturday. But more than 4 million people remain without electricity along the Eastern Seaboard.

An enormous job lies ahead. Irene flooded power stations, toppled trees and even threatened to close the New York Stock Exchange.

Repair crews, with thousands of workers, rushed out in Virginia, the Carolinas and Maryland Sunday. But as lights switched on in the South, many more went dark in the cities of the Northeast as the storm churned north.

"A number of rivers in northern New Jersey are under an extreme flood watch," said Ron Morano, a spokesman for Jersey Central Power & Light. "The number of outages are going to go up today."

In Manhattan, Consolidated Edison said it was optimistic that it wouldn’t need to cut power to the financial district. So far, sensitive underground power lines haven’t been flooded, the company says.

Cutting power to the financial district could leave Wall Street institutions like the New York Stock Exchange on backup generators until the middle of the week.

Power companies say they’ll focus on parts of their system that can restore power to the most people at once. They’ll start with massive transmission lines that supply entire counties before turning to smaller problems within individual neighborhoods.

Utilities in southern Pennsylvania, North Carolina, Virginia and Maryland say they’ve restored power to more than 600,000 people as of Sunday morning.

After pummeling those states, Irene whipped northward. But it was downgraded to a tropical storm after its winds slowed on Sunday and it continues to lose strength as it passes over New England. It’s expected to move into eastern Canada by Sunday night.