The storm, which swept inland early Monday, tore holes in the roof of the Superdome where some 10,000 people had sought shelter.
Louisiana state Sen. Walter Boasso, R-Chalmette, said 12 feet of water inundated St. Bernard Parish, reaching to the second floor of many homes.
Spokesman Larry Ingargiola said shelters at Chalmette and St. Bernard high schools suffered major damage, WWL-TV reported. CNN reported the parish's 911 center was destroyed by the storm and the parish was mostly without power.
Homeland Security Chief Terry Ebbert said it may be a month before power is fully restored.
City Councilman Oliver Thomas reported receiving 120 distress calls from people trapped by the storm but crews were unable to get to them while the winds raged.
An apartment building in Terrytown collapsed -- one of a number of structures that failed in the storm's onslaught.
WWL-TV reported patients huddling in the halls of Charity Hospital, where windows were blown out on the fourth floor.
NBC said the hurricane's 145 mph sustained winds ripped two 7-foot-by-7-foot vents off the Superdome's roof, creating gaping holes. Numerous other holes dotted the dome.
The Louisiana National Guard began moving people into ancillary areas and officials said they did not expect the building to fail.
The Mississippi River was reported to have risen 15 feet since Sunday night.
Trees were uprooted and aluminum siding was peeled off houses in Baton Rouge, Lacombe and elsewhere.