NEW YORK — The Atlantic superstorm Sandy churned across Pennsylvania Tuesday after blacking out much of southern Manhattan and leaving a trail of flooding, death and destruction along the East Coast.

Government offices and stock markets were shut for a second day amid damage that may total billions of dollars. The Associated Press reported at least 20 deaths related to the storm, and New York Mayor Michael Bloomberg said 10 people were dead in the city.

"It's fair to say the path of destruction that she left in her wake is going to be felt for quite some time," the mayor said at a press conference Tuesday. "Make no mistake about it, this was a devastating storm, maybe the worse that we have ever experienced." He said he expects the number of deaths to go up as more information comes in.

Subways will be out for several days after several tracks and stations flooded, he said. Limited bus service will start today. About 750,000 New Yorkers were without power.

From Washington D.C. to Boston, transit systems, businesses and homeowners awoke to assess the damage and begin recovery. The storm, which came ashore Monday evening in southern New Jersey, brought life-threatening floods to a region with 60 million residents. It interrupted the U.S. presidential race eight days before Election Day. President Barack Obama declared New York and New Jersey disaster regions eligible for federal relief.

The storm may cause as much as $20 billion in economic damage and losses, according to Eqecat Inc., a risk-management company in Oakland, Calif.

Sandy weakened with its center drifting west through central Pennsylvania. Its core was about 120 miles (193 kilometers) east-southeast of Pittsburgh with maximum sustained winds of 45 miles per hour, according to an 11 a.m. advisory from the U.S. Hydrometeorological Prediction Center in Camp Springs, Maryland.

More than two feet of snow fell in western Maryland and nearly as much in West Virginia, according to the advisory.

"For the major cities of the east the worst of the weather is done," said Tom Kines, a meteorologist at AccuWeather Inc. in State College, Pa. "Eventually, over the next 24 hours, the system will rain itself out."

New York City may have some rain showers later today while overall the weather gradually improves, Kines said. The weakened storm will move into southern Canada later this week.

The storm left 8.11 million electricity customers without power in 17 states and the District of Columbia, from South Carolina to Maine and as far west as Michigan and Indiana, according to the Department of Energy, including 62 percent without power in New Jersey, and 31 percent in Connecticut.

Power was lost in Manhattan "river to river," south of 35th Street, Bloomberg said Monday night. Some of the blackout was deliberate, as Consolidated Edison Co. shut off electricity to protect its underground equipment from potential damage, said Chris Olert, a spokesman for the company.

The mayor this morning issued a message urging city employees to report to work and help with recovery operations if they could do so safely.

A flood gauge at Battery Park, at the southernmost end of Manhattan, registered at 13.88 feet at 9:24 p.m. Monday, beating the modern record of 10.02 feet in September 1960 during Hurricane Donna, the National Weather Service said.

New York University's Langone Medical Center evacuated 215 patients, including infants from its neo-natal intensive care unit, and transported them to other hospitals when it lost power and backup systems failed.

A fire in Breezy Point in the New York City borough of Queens had about 198 firefighters working to contain a blaze that had destroyed at least 50 homes and left two people with minor injuries, a New York Fire Department official said Tuesday.

Sandy forced three nuclear power plants to shut and put another on alert as federal regulators dispatched inspectors to monitor 11 facilities in the path of the storm.

The nation's oldest nuclear power plant, Exelon Corp.'s Oyster Creek facility in New Jersey, declared an alert last night due to elevated levels of water in its water-intake structure, the Nuclear Regulatory Commission said in a statement.

Insured losses may exceed $6 billion, led by costs in Pennsylvania, New Jersey, Maryland and New York, according to estimates from Kinetic Analysis Corp. compiled by Bloomberg. As many as 10 million people may lose power, according to Seth Guikema, a Johns Hopkins University engineer.

Airlines grounded 6,047 of U.S. flights, or 20 percent of those scheduled for today. About 12,500 trips were canceled from Oct. 28 until today, according to FlightAware, a Houston-based tracking company.

Washington's transit authority said limited Metro rail and bus service would start at 2 p.m. today, with full service resuming Wednesday. Downed trees and power outages might cause delays or detours on some bus routes, it warned.

All U.S. equity markets, which were closed yesterday, were shut again today, the first shutdown for consecutive days due to weather since 1888. The Securities Industry and Financial Markets Association said bond trading would be suspended today.

The floor and building of the New York Stock Exchange "are fine" and the company is working toward restoring normal trading, according to Robert Rendine, a spokesman. NYSE Euronext's building on Wall Street is close to the section of Manhattan that was flooded when the storm propelled a 13-foot sea surge as it came ashore Monday night.

Phillips 66, NuStar Energy and Hess Corp. shut or reduced output at three New Jersey refineries ahead of the storm's landfall. At least three other plants were running at reduced rates.

In New York, Mayor Bloomberg, the founder and majority owner of Bloomberg News parent Bloomberg LP, Monday issued evacuation orders for 375,000 people and opened 76 shelters.

On 57th Street, a crane on a 90-story residential building under construction partially collapsed and was dangling over the street near Carnegie Hall. No injuries were reported.

With Lower Manhattan almost a ghost town for a second day, seawater cascaded into the still-gaping construction pit at the World Trade Center, the AP reported Tuesday.

Off North Carolina's Outer Banks, one person was killed and another was missing after the crew of the HMS Bounty, a replica of the vessel that was the scene of a 1789 mutiny, abandoned ship when it capsized in 18-foot seas.

In New Jersey, Gov. Chris Christie ordered evacuations of coastal barrier islands and casinos in Atlantic City, which was flooded. A large number of people were stranded as water levels rose. Christie said further evacuations from Atlantic City and the barrier island would be impossible until daylight.

In Connecticut, New London Mayor Daryl Finizio told WFSB television that Sandy was worse than the Great New England Hurricane of 1938. That storm, which produced tides of as much as 25 feet, killed 564, according to the National Oceanic and Atmospheric Administration. New Haven radio station WTNH reported two state residents, a firefighter and an 80-year-old woman, killed from falling trees.

"Thousands of people are stranded," said Gov. Dannel Malloy at a news briefing Monday night.

In Maryland, Gov. Martin O'Malley said the state was fortunate to escape the worst blows of the storm. There were two fatalities — one caused by a traffic accident, the second by a felled tree — and the state will continue to face flooding in areas over the next two days, he said.

"We were not hit as hard as all of us had anticipated," he told reporters today. "We were fortunate to have been on the weaker end of this storm."

The storm also had an impact on the Nov. 6 presidential election, forcing Obama and Republican challenger Mitt Romney to cancel appearances.

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With assistance from Alex Wayne, Alan Levin, Sandrine Rastello, Julie Hirschfeld Davis, Jim Snyder, Hans Nichols, Emma Fidel, Kasia Klimasinska and William Selway in Washington, Phil Milford in Wilmington, Delaware, Elise Young in Trenton , John McCormick in Chicago, Peter S. Green, Margaret Collins, Stephen Merelman and Esme E. Deprez in New York and Rupert Rowling in London.