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NEW ORLEANS, Sept 2 (Reuters) - A mandatory evacuation of New Orleans declared in advance of Hurricane Gustav will end at midnight on Wednesday, allowing residents to return on Thursday, Mayor Ray Nagin said.

Nagin said on Tuesday it would take time before all of the city's hospitals, water and sewage services, and power systems were fully operational and he urged residents to be cautious in returning.

"The message is: We want you to come into the city, check on your property, make sure that you are comfortable and make an intelligent decision on whether you want to stay in this environment or not," Nagin told a news conference.

"At midnight on Wednesday we will go from a mandatory to a voluntary evacuation status."

New Orleans remained in a "vulnerable state," Nagin said, after what he described as a "stealth storm" that damaged the region in ways that were not as visible as the devastation caused by Hurricane Katrina three years ago.

Some 1.9 million people fled the Gulf Coast ahead of Gustav, the largest such evacuation in U.S. history. Officials credited the evacuation with saving lives.

Gustav hit the Gulf Coast southwest of New Orleans on Monday, packing 110 mph (177 kph) winds.

A rebuilt levee system around New Orleans protected the city from the kind of flooding that followed Katrina.

But most of the city remains without electricity, with hospitals and its treatment system running on backup power and generators.

Electric utility Entergy Corp said all 14 high-power transmission lines taking power into New Orleans had been knocked out by Gustav and only one had been repaired. Officials said it was not clear when power could be restored. (Editing by Kevin Krolicki and John O'Callaghan)

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