Sandy, the hybrid hurricane/nor’easter, began to lose steam Tuesday as it drifted across Pennsylvania and veered toward Canada. But the damage was done, and it will go down as a historic storm, not least because of what it did to New York, where a surge of seawater inundated some of the most valuable real estate in America.

Much of Manhattan, the seat of American finance, is in the dark. Someone standing after dusk Tuesday in the middle of the Brooklyn Bridge would see the lighted-up Chrysler Building and other Midtown skyscrapers to the north but darkened buildings to the south — almost all of Lower Manhattan vanishing into the night. Only City Hall was illuminated.

Power could be out for a week — a fact noted by some New Yorkers who packed their bags and headed for the exits.

The storm was blamed for 48 deaths up and down the East Coast, according to the Associated Press. The tempest played havoc with the power grid, knocking out electricity to 7.5 million people. More than 16,000 airline flights have been canceled so far. Eqecat, a firm that models the costs of catastrophes for insurance companies, estimated Sandy’s economic impact on the country at $10 billion to $20 billion.

At the point of attack was the New York City, a marvel of infrastructure and civil engineering that rediscovered this week that it is a coastal city, and that nature can be vicious. Sandy’s high winds sparked fires that destroyed scores of houses. All the New York City airports remained closed Tuesday, along with the flooded subway. Wall Street never opened for business, the first two-day closure due to weather since the days of horses and buggies. The United Nations will be closed Wednesday for the third straight day.

“The damage we suffered across the city is clearly extensive, and it will not be repaired overnight,” said New York Mayor Michael R. Bloomberg.

Bloomberg put the city death toll at 10. The toll could have been higher: Firefighters rescued 25 people from an upstairs apartment as they battled a huge blaze in the Breezy Point neighborhood of Queens. Another drama unfolded late Monday at New York University’s Tisch Hospital, when a backup electrical system failed and nurses had to evacuate infants from neonatal intensive care, carrying them down darkened stairwells to get them to the safety of another hospital.

The Breezy Point fire immolated 80 homes, one of which belonged to a congressman, Robert L. Turner, R-N.Y.

In Brooklyn, Dave Shamoun, 58, the owner of Technico Marine, a marine industry supplier, surveyed the soggy wreckage in a 15,000-square-foot warehouse.

“This is New York’s Katrina,” Shamoun said.

Some residents of Sheepshead Bay, an old fishing community in southern Brooklyn, tried to ride out the storm in their wood-frame houses and bungalows. They were inundated by fierce waves that surged in from Manhattan Beach. Water ripped apart a 100-year-old esplanade and yanked sailboats from their moorings. Mud and water invaded storefronts and shattered the plate-glass windows at Tete a Tete Cafe on Avenue Z.

“It didn’t seem as if anyone had prepared their homes before the storm came in,” said Ned Berke, editor of Sheepshead Bites, a news website that covers the neighborhood. “They thought it was going to be like Irene.”

The denizens of Lower Manhattan were astonished by the sight of submerged and floating cars in vintage residential neighborhoods and the financial district. In the East Village, more than a dozen people waited on line Tuesday at the Village Farms grocery store, where workers escorted each shopper for a flashlit tour of the aisles. Around the corner, 37 people waited in line at a coffee truck, some amusing themselves by guessing how long it would be before power returned to their apartments.

“Four days? Thanksgiving? ” asked Zack Zavada, 29, a clothing salesman who said he had seen a Con Edison transformer explode from his window.

At Fanelli’s, open since 1847 at Prince and Mercer Streets, there were no empty stools as the bartender served by flashlight. Small candles burned on the bar.

“Liquor, no food,” said Mark Michaelson, 56, an art director, taking a smoke break at the entrance. “A Jameson’s is like a sandwich.”

Every block in Lower Manhattan offered a different version of a city in its first full day of coping with Sandy’s effects. In TriBeCa, people walked from corner to corner with their smartphones searching for Internet service. Chances improved with every block northward, so many headed north by foot.

At Bleecker Street Pizza in the West Village, one of the first restaurants to reopen, dozens of people waited in line for a warm slice. Most had endured the night without power, and looked it. The smell of yeast blasted down the sidewalk, pushed by wind gusts. The fiercest winds were gone, but the rain continued, and as it intensified nobody budged, including Jonathan Padron, 26, who came out with a box of sausage pizza after a harrowing night of darkness and a tin of tuna in his apartment.

“I was so stressed out, I had to meditate,” said Padron, a dog walker who works in Brooklyn.

Businesses, apartment dwellers and homeowners from Manhattan’s East Village to Brooklyn’s Red Hook spent Tuesday pumping floodwaters from their basements and trying to salvage waterlogged possessions. Simply crossing the street was dangerous, with streetlights out and traffic cops scarce.

Elizabeth Freund, 49, returned to her home in Red Hook on Tuesday morning to find it inundated by nearly 3 1/2 feet of water. “My bedroom is floating, my office is floating, my daughter’s room is floating,” she lamented.

Gino Vitale, a Brooklyn landlord who owns 25 apartments in the area, said that 16 of them were flooded, some submerged in more than eight feet of water. One of his renters phoned him in a panic about 7 p.m. Monday, saying, “What do I do?” Get out, he answered. “By 9 it was over the fridge,” he said.

People were stunned at the sight of a Bayliner pleasure boat that was swept into the very end of Sheepshead Bay, slamming into the concrete abutment. The smell of gas oozed from its tank.

“What’s that smell?” cried Bella Kharajyan. Her daughter, Milena Rangini, 27, covered her nostrils with her scarf. The two had spent a long, tiring night in their second-floor apartment in Brighton Beach as other residents flocked to their door, knowing that the two women from Armenia spoke English and could understand the news reports.

Kharajyan bemoaned the lack of help for people who speak languages other than English and Spanish, such as their Armenian- or Uzbek-speaking neighbors. “A little bit I understand American information news,” she said. “I don’t see anybody helping us.”

President Barack Obama signed federal emergency declarations for 10 states and the District of Columbia, and he canceled campaign plans for Monday and Tuesday so he could remain at the White House and oversee the storm response. After visiting the headquarters of the Red Cross in Washington, Obama told reporters, “My message to the federal government: No bureaucracy. No red tape.” He said if local officials get no for an answer from the federal government, “they can call me personally at the White House.”

Republican challenger Mitt Romney also shelved many of his campaign plans but held a “storm relief” event near Dayton, Ohio. Romney ignored repeated questions from reporters about whether he wished to scale back the Federal Emergency Management Agency, a position he advocated during a GOP primary debate.

Obama will visit New Jersey on Wednesday, touring damage with Gov. Chris Christie, a Republican with his own presidential aspirations. Christie said early Tuesday that 2.4 million New Jersey households were without power, twice the number that lost electricity during Irene.

Later in the day Christie toured the coastal towns by helicopter.

“I was just here walking this place this summer, and the fact that most of it is gone is just incredible,” Christie said to Belmar Mayor Matt Doherty, according to an Associated Press pool report.

The governor vowed to rebuild:

“This is the kind of thing New Jerseyans are built for — we’re plenty tough, and now we have a little more reason to be angry after this,” he said. “Just what we need in New Jersey — a chance to be a little more angry.”

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Lynch reported from New York. Paul Schwartzman, Anne Hull, Suzanne Sataline and Katie Van Syckle in New York; Lisa Rein in New Jersey; and Dan Eggen and William Branigin in Washington contributed to this report.