Hurricane Isaac drove water over the top of a levee on the outskirts of New Orleans on Wednesday, but the multibillion-dollar barriers built to protect the city itself after the 2005 Katrina disaster were not breached, officials said.

The slow-moving but powerful Category 1 hurricane was felt along the Gulf Coast, threatening to flood towns in Mississippi and Louisiana with storm surges of up to 12 feet and top sustained winds up to 75 miles per hour (120 kilometers per hour).

"The federal levee system … is fine," New Orleans Mayor Mitchell Landrieu told local radio.

"There are no risks. It is holding exactly as we expected it to and is performing exactly as it should. There are no people on rooftops from flooding that even approximates what happened during Katrina," he said.

Police and National Guard units, many wielding automatic assault rifles, patrolled the virtually empty downtown quarter of the port city, which normally bustles with tourists drawn to its jazz bars, Creole cuisine and French colonial architecture.

Tree limbs and street signs littered the streets and power was cut intermittently throughout the city, but authorities reported no security problems.

"Thus far it's been pretty easy," said Captain Jeremy Falanga of the Louisiana National Guard, who was stationed with troopers in front of the city's convention center. "Not many people are outside, it's pretty buttoned up."

In low-lying Plaquemines Parish, which stretches southeast from New Orleans, emergency officials reported the overtopping of an 8-foot (2.4-meter) high levee between the Braithwaite and White Ditch districts.

Plaquemines Parish President Billy Nungesser said about 2,000 residents of the area had been ordered to evacuate but only about half were confirmed to have gotten out before Isaac made landfall late on Tuesday.

Isaac was wobbling northwestward near six mph, a slow pace that increases the threat of rain-induced flooding.

"On the east bank right now, we have reports of people on their roofs and attics and 12 to 14 foot of water," Nungesser told CNN.

"This storm has delivered more of a punch than people thought," he added.