Tropical storm Isaac continued to lash New Orleans with heavy rains today after being downgraded from hurricane status.

A newly-fortified levee system appeared to be holding exactly seven years after Hurricane Katrina devastated the city.

In a drenched part of rural Louisiana, rescuers on boats helped dozens of people stranded by floodwaters.

In New Orleans, power lines were downed and debris littered the streets, prompting officials to impose a dusk-to-dawn curfew.

Louisiana officials said they may have to intentionally breach a levee in a flooded area as Isaac made slow, drenching progress inland from the Gulf of Mexico.

In a hard-hit rural area southeast of New Orleans, officials rescued dozens of people by boat, while authorities feared more in Plaquemines Parish could need help after a night of slashing rain and fierce winds that knocked out power to more than 700,000 households.

This evening Isaac was sustaining winds of 112kph, just below the hurricane threshold of 119kph.

Louisiana Governor Bobby Jindal said officials may cut a hole in a levee on the east bank of Plaquemines Parish to relieve pressure on the structure.

At a news conference in Baton Rouge, Mr Jindal said there was no estimate on when that might occur. He said as many as 40 people are reportedly in need of rescue in the area.

Plaquemines Parish has also ordered a mandatory evacuation for the west bank of the Mississippi River below Belle Chasse, over fears of a storm surge. The order affects about 3,000 people in the area, including a nursing home with 112 residents.

Officials said the evacuation was ordered out of concern that more storm surge from Isaac would be pushed into the area and levees might be overtopped.

The hurricane also forced the cancellation of commemoration ceremonies for Katrina's 1,800 dead in Louisiana and Mississippi.

Isaac was testing a New Orleans levee system bolstered by $14bn in federal repairs and improvements after the catastrophic failures during Katrina.

Army Corps spokeswoman Rachel Rodi said the city's bigger, stronger levees were withstanding the assault.

"The system is performing as intended, as we expected," she said. "We don't see any issues with the hurricane system at this point."

New Orleans Mayor Mitch Landrieu issued a curfew for the city as Isaac lashed the city on the seven-year anniversary of Katrina's destructive arrival.

Police cars had been patrolling the nearly empty streets since Isaac began bringing fierce winds and heavy rains to the city last night.

The curfew was set to start tonight and would last until further notice.