PHILADELPHIA — Residents in eastern Pennsylvania hunkered down for a winter storm on the day after Christmas that prompted Philadelphia and other cities to declare snow emergencies and forced cancellation of hundreds of flights as well as postponement of the Vikings-Eagles game.

Mayor Michael Nutter declared a snow emergency in Philadelphia beginning at 2 p.m. Sunday and urged residents to stay off roads. Forecasters projected 6 to 10 inches of snow along with strong winds that would make for dangerous driving conditions.

“We are urging all Philadelphians, please be careful, please be safe,” the mayor told reporters at City Hall. Nutter urged residents and travelers to get an early start and be off the roads by the time the storm hit because “when it happens, it will hit very hard.”

At Philadelphia International Airport, about 400 flights had been canceled by Sunday afternoon, about two-thirds of the anticipated arrivals and departures, spokeswoman Victoria Lupica said. She said a hospitality program was ready to assist stranded travelers, especially those arriving later on international flights. US Airways also said it was canceling 110 flights today, mainly in Northeast cities such as Philadelphia, New York and Boston.

The Southeastern Pennsylvania Transportation Authority vowed to keep buses, subways, trolleys and regional rails operating for as long as possible but said significant delays could be expected on some lines if the storm progressed as expected.

The National Weather Service issued a winter storm warning through 1 p.m. today, initially projecting 8 to 12 inches of snow in Philadelphia and its suburbs but later said 6 to 10 inches was expected, the same as for other areas of eastern Pennsylvania to the north. A winter weather advisory projected 2 to 6 inches for central and Western Pennsylvania.

The Pennsylvania Department of Transportation said speed limits on interstates and expressways had been reduced from 55 to 45 mph as of about sundown due to reduced visibility. Officials said trucks had been out the night before pouring brine on major highways, and 410 trucks would be salting and plowing the roads during the storm.

Other municipalities in suburban Philadelphia and eastern Pennsylvania also declared snow emergencies, asking residents to move any parked vehicles off snow emergency routes. More than 200 trucks were deployed to salt and clear roads in the six-county area around the Lehigh Valley north of Philadelphia.