Mayor Nutter declared the first state of emergency in Philadelphia since 1986. He warned that some residents could lose power for more than a week, and said the Schuylkill was expected to crest at 8 feet above normal. That would be several feet above flood stage and a level, he said, that has not occurred since 1869.
By early evening, flooding already had begun along Cobbs Creek in Southwest Philadelphia, Nutter said.
"This is one of the worst storm events that has hit Philadelphia in the last 50 years," he said.
More than 200,000 business and residents throughout the region had lost power by late Saturday, utilities reported.
Tornado warnings were issued late Saturday in Philadelphia and other parts of the region. Twisters did touch down in Vineland, N.J., and near Lewes, Del., where fire officials reported one house was demolished.
Nutter's emergency declaration came toward the end of a day - several days, really - of historic warnings and preparation. Still, Irene inched north Saturday at an unusually slow pace, about 13 m.p.h., creating a tense daylong countdown that left people unsure what to expect - or when.
Early downpours turned to mist, then back to rain. Towns activated emergency plans, malls closed, and trains and planes stopped running. Philadelphia International Airport expected to repoen at 4 p.m. Sunday.
At the same time, some treated the hurricane with indifference, deciding it was hyped and not worth changing their routine.
In Manayunk, a neighborhood accustomed to Schuylkill floods, some residents stacked sandbags in front of their homes as others sipped coffee at sidewalk cafes or jogged along the river trail. A wedding reception went on.
The Phillies opened the gates at Citizens Bank Park to 12,000 fans for a 1:05 p.m. game, then canceled the game and sent them home minutes after the first pitch was to have been thrown.
More than one million people obeyed warnings to flee the New Jersey coast, but officials struggled to persuade thousands more who refused to move inland.
"Units are driving around making the announcement: As soon as the storm truly hits, you're on your own," said Michael Cahill, a fire department captain in Ventnor, where he estimated 1,100 people had ignored the evacuation order.
In Cape May, residents who did not heed the evacuation were being told Saturday night to stay put, said Lenora Boninfante, county communications director.
Earlier in the day, Gov. Christie had dispatched buses and personnel to a half-dozen Atlantic City high-rises to coax 600 residents - mostly seniors - from their units and into shelters off the island.
At one site, the Best of Life senior apartments, 76-year-old manager Dorothea Arlotta said she "couldn't care less" about Christie's edict.
"We are absolutely safe," Arlotta insisted. "When you get older, creature comforts are very important. . . . We're all in here together, and we take care of each other."
Officials repeatedly warned of the storm's potential. With tropical winds stretching across 290 miles, Irene was a monstrous storm on a steady march up the I-95 corridor - the country's most densely populated stretch and one unaccustomed to major hurricanes.
The storm was expected to land its biggest punch in the dead of night, lingering into Sunday afternoon. Nutter said many residents would likely wake up to find their power out and basements flooded.
Up to a foot of rain was forecast for the region.
Winds were likely to top 55 m.p.h., with hurricane-strength gusts of 75 m.p.h. along the coast. The storm would weaken slightly as it made landfall, but forecasters said they expected it to maintain hurricane strength as it neared New York on Sunday.
Irene arrived at the end of the wettest month on record for the region. The saturated ground meant trees were more likely to easily topple and rivers and creeks would quickly rush over their banks.
In Burlington County, officials worried that the Rancocas Creek could surge as it did after a 2004 storm - when 21 dams failed, displacing 400 families and causing $17 million in damage.
"The prospects for flooding are really strong - there's no getting around it," said county spokesman Ralph Shrom.
South of New Jersey, more than two million people along the Eastern Seaboard already had evacuated to escape Irene, and nearly a million had lost power. At least five deaths were attributed to the hurricane, among them a North Carolina man killed by a falling tree limb and a Virginia boy who died when a tree fell on his family's apartment.
Across Pennsylvania and New Jersey, dozens of towns activated emergency plans as the first drops fell.
Delaware County officials evacuated the low-lying areas of Darby, Eddystone, and Chester. Shelters opened from Radnor Township in Delaware County; to Cheltenham, West Norriton, and Pottstown in Montgomery County; to Glassboro in Gloucester County.
In Glassboro, more than 1,100 people packed an American Red Cross shelter at Rowan University's gymnasium.
As children napped or scrawled in coloring books, Avis Newmones and her nephew, Aaron, sat on folding chairs and played cards with two other evacuees.
"We've got to find something to do to pass the time," Newmones said.
She was one of the thousands who had fled Atlantic City, turning that and many Shore points into virtual ghost towns on a late summer weekend.
Most major roads to the Jersey Shore were closed to eastbound traffic, and dozens of beach towns ordered residents to evacuate. Even 30 Wawa stores - the last line of defense for people in search of storm supplies - closed in Cape May, Ocean, and Atlantic Counties.
But not everyone left.
In Atlantic City, Charles Collins, 58, watched his girlfriend leave but said he planned to ride the storm out in his fourth-floor apartment.
"I'll take out my AC and shore up the windows," he said. "I don't think it's going to be that bad."
In neighboring Margate, Leo Heintzelman was conducting business as usual.
"We never close," said Heintzelman, owner of Dino's Sub Shop, as he sold subs to go. "We've been open since 1960, and the rule is you close Thanksgiving and Christmas - and that's it."
In North Wildwood, torrential rains started flooding some streets as early as 1 p.m. as police, firefighters, and elected officials gathered to discuss emergency plans. Mayor Bill Henfey said many people had volunteered to stay in town, including lifeguards.
"We need people who can swim - well," he said.
In other areas, shelters were filling up.
In Philadelphia, the Red Cross opened shelters at Lincoln, Bartram, and Roxborough High Schools. In Camden, shelters at the Malandra Hall and Isabel Miller community centers will remain open until 8 a.m. Monday, according to city officials.
By then the storm is expected to have given way to nice weather. But the aftermath of Irene will linger.
Some schools scheduled to be open Monday were already canceling classes. The University of Delaware, among other colleges, said it would postpone the start of the school year for freshmen.
In Pennsylvania, Gov. Corbett said runoff from North Jersey and New York would surge into the Delaware River for days.
"This event may not be just a 24-hour event when you consider the flooding," he said. "The river may not crest until sometime Tuesday or Wednesday."
Such predictions were what stirred Dan Margo, owner of Peter Wallace Ltd., an antiques store on the Delaware in Lambertville, N.J., to haul hundreds of his items to higher ground.
"If you think it's going to flood, you just do it," Margo said.
Contact staff writer John P. Martin
at 215-854-4774 or at email@example.com.
Contributing to this article were Inquirer staff writers Amy S. Rosenberg, Maya Rao, Sally Downey, Jennifer Lin, Faye Flam, Jaqueline L. Urgo, James Osborne, Bonnie Cook, and Joseph DiStefano, and Philadelphia Daily News staff writer Jason Nark.