By Jennifer Weiss and Lisa Fleisher

NORTH MIDDLETOWN, N.J.--Gov. Chris Christie declared a state of emergency in anticipation of Hurricane Sandy, warning that parts of New Jersey could be without power for days.

Mr. Christie told residents of a state weary from storm warnings not to underestimate Sandy, which is expected to be "in full force" by Monday. He ordered evacuations by 4 p.m. Sunday of New Jersey's barrier Islands and the Atlantic City casinos.

At a news conference Saturday at the East Keansburg Fire Department, he told a crowd of residents, press and elected officials that Sandy was "expected to be a severe, potentially devastating storm" on track to hit New Jersey beginning Sunday night "and be in full force by Monday."

He urged residents to take the storm seriously, saying it could be more severe than Hurricane Irene, which caused major flooding and power outages when it made landfall as a tropical storm in New Jersey in August 2011. Power companies said New Jerseyans could be without power for seven to 10 days, he said.