With floodwaters relentlessly flowing into this low-lying city from breached levees in the wake of Hurricane Katrina, federal, state and local authorities launched an emergency repair effort Tuesday aimed at closing the gaps and, eventually, pumping out the water.

The U.S. Army Corps of Engineers reported Tuesday afternoon that an assessment team had found breaches in two canal floodwalls in the central and eastern parts of New Orleans, much of which lies below sea-level.

Louisiana Gov. Kathleen Babineaux Blanco (D) told a news conference that authorities are working on a plan to evacuate the residents who remained in New Orleans despite a mandatory evacuation order issued Sunday as Katrina, then a Category 5 hurricane, bore down on the Gulf Coast.

Compounding the city's woes were looting and the spread of pollutants borne by the spreading floodwaters.

State and city officials were initially relieved when Katrina weakened to a Category 4 storm and delivered its central blow about 63 miles southeast of New Orleans when it made landfall Monday morning, sparing the city the huge storm surge of up to 28 feet that had been feared. Initially, the levees that protect the city appeared to have largely held.

But the sense of relief faded Monday night and Tuesday morning, replaced with growing dread because of breaches in the levees that keep out water from Lake Pontchartrain just north of New Orleans. Officials and residents feared that the floodwaters could have the same catastrophic effects as the storm surge, even though they were unfolding more slowly.

The Corps of Engineers said in a statement that the city's 17th Street Canal floodwall was breached overnight following Katrina's passage and that another breach occurred on the Industrial Canal during the storm.

Closing the gaps is "essential so that water can be removed from the city," the statement said.

"The Corps is working with the U.S. Coast Guard, Army National Guard and other state and federal authorities to bring in all assets available to expedite the process," the statement said. Walter Baumy, Engineering Division chief and project manager for closing the breach, said the Corps was attempting to contract for materials such as rock, super-sized sand bags, cranes, barges, helicopters and other equipment "to close the gap and stop the flow of water from Lake Pontchartrain into the city."

New Orleans has 350 miles of hurricane levees built to withstand a fast-moving Category 3 storm, the Corps said. "The fact that Katrina, a Category 4-plus hurricane, didn't cause more damage is a testament to the structural integrity of the hurricane levee protection system," the statement said.

In Baton Rouge, a National Guard commander told reporters that the breach in the 17th Street Canal floodwall was almost 300 feet long. The one in the Industrial Canal floodwall is reportedly much smaller.

He said one proposal is to fill large shipping containers with sand and insert them into the breach to plug it.

Gov. Blanco told a late-afternoon news conference that thousands of people have been rescued from flooded homes and rooftops, "and there are many more that have to be saved."

"The volume of the work is incredible," she said. "I just think our people are just going to have to draw on their inner strength." Some neighborhoods will require "total rebuilding," she said. "Many buildings are totally devastated. . . . Some are in shards."

Parts of Interstate 10 look "like a jigsaw puzzle," with sections missing or out of alignment, Blanco said.

Many of the people who needed to be rescued were residents trying to return to their homes, she said.

As seen from a helicopter, "most of the roads and highways are impassible, and water is still coming into the city of New Orleans," said Sen. Mary Landrieu (D-La.). She said the water was "up to the rooftops" in some parishes.

Said Blanco, "There's water lapping at the foot of the Superdome now." Generators at the huge stadium, which was designated a refuge of last resort as the storm approached, are being jeopardized by the rising floodwaters, she said.

At Tulane University Hospital, officials were trying to evacuate about 200 patients, including 60 who were brought to the hospital from the Superdome during the storm.

"If the water continues to rise, we will lose all our backup generator power in the building," hospital spokeswoman Karen Troyer Caraway told CNN.

Blanco said the first priorities are to rescue people and bring in enough supplies to sustain those who are still in New Orleans. Then, she said, they will be evacuated.

Water levels in Lake Pontchartrain and the connecting 17th Street Canal are normally six feet higher than the surrounding city. The levees keep the waters from flowing down into this low-lying city, much of which is below sea level.

The damage to the 17th Street Canal and its levee means that the water from Lake Pontchartrain is now free to flow down to inundate hundreds of thousands of homes and other buildings here.

Once it flows in, the water will not drain from New Orleans because of the very levees that protect the city and that largely held during the hurricane. Those levees, built to keep water out, are now keeping the water in, and reports from across the city indicate that water levels are rising.

As the floodwaters rose, looting was reported -- some of it in full view of National Guard troops and New Orleans police. Amid the flooding, looters raided clothing, jewelry, grocery and drug stores, sometimes filling garbage cans and floating them away on pieces of wood and other building materials in waist-high waters.

"It's downtown Baghdad," said Denise Bollinger, a tourist from Philadelphia, as she watched looting in the French Quarter, the Associated Press reported. "It's insane. . . . I thought this was a sophisticated city. I guess not."

"We're getting reports of sporadic looting," said Lt. Lawrence McLeary, a Louisiana State Police spokesman. The looting began Monday and was continuing Tuesday in a number of areas that are "inaccessible from the outside," he said in a telephone interview from Baton Rouge.

Even in areas that can be reached by New Orleans police, "it's pretty difficult for them to make any arrests, because there's no place to facilitate those arrests," McLeary said. He said police "are just trying to stop that looting and get people out of there."

In fact, a New Orleans jail had to be evacuated, and prisoners in jumpsuits were being guarded in the open on a nearby highway cloverleaf.

In addition to a state of emergency declared by the governor, McLeary said, some parishes in and around New Orleans have declared their own states of emergency, allowing local law enforcement officials to impose curfews and prohibit public access to certain areas.

There was also growing concern that the floodwaters were carrying sewage, spilled fuel and other potentially dangerous pollutants.

Also, bodies were seen floating in the rising waters, New Orleans Mayor Ray Nagin reported. But it was not immediately clear whether they were victims of Katrina or corpses unearthed from the above-ground tombs of the city's cemeteries.

New Orleans normally uses pumps to get the water out when necessary, but the city has been without power since the hurricane struck with 140-mph winds around daybreak Monday.

It is difficult to know how many people are threatened because of the mass evacuation before Hurricane Katrina. A caller to a local radio station reported that the floodwater in her New Orleans home was rising and that she could not swim. Boating is rapidly becoming the best way to travel here.

The levee damage was first noticed during an assessment flight Monday afternoon, but its extent and significance were not immediately understood. By late Monday, the rising water levels here made its significance apparent.

The rising floodwaters in the city of 485,000 people were preventing residents from returning to their homes.

Mayor Nagin estimated earlier that about 80 percent of the population heeded the evacuation order, leaving more than 90,000 people in the city when the hurricane hit.

At the Superdome, designated by Nagin as one of 10 refuges of last resort for people who were unable to evacuate, National Guard troops allowed dozens of refugees to sleep on the walkway surrounding the huge building as conditions inside deteriorated, but authorities refused to let them leave.

As many as 10,000 people took shelter in the Superdome starting Sunday when Nagin ordered the mandatory evacuation of the city. As the hurricane struck Monday morning, the high winds tore off much of the outer skin covering the Superdome's 9.7-acre roof and punched two holes clear through it, allowing rainwater to leak in.

By Tuesday, bathrooms were filthy, trash barrels were overflowing and stadium aisles and steps were slick with humidity because of the lack of air conditioning since the power failed during Katrina's onslaught. Under those conditions, some of the refugees were allowed to take their bedding out onto the concourse to cool off and breathe some fresh air.

One group was dismayed to hear on a newscast that authorities in suburban Jefferson Parish were not allowing residents to return until next Monday, the Associated Press reported. The group groaned, and one woman cried.

"I know people want to leave, but they can't leave," said Gen. Ralph Lupin, a National Guard commander at the Superdome, the AP reported. "There's three feet of water around the Superdome."

Doug Thornton, a regional vice president for the company that manages the Superdome, said two people have died there, the news service reported. He did not provide details.

"The city of New Orleans is in a state of devastation," Mayor Nagin told local television station WWL last night. "We probably have 80 percent of our city under water," in some places as deep as 20 feet. He said both airports were under water, and people were on roofs awaiting rescue.

Michael D. Brown, head of the Federal Emergency Management Agency, warned in an interview on CBS that residents may not be able to return to their homes anytime soon. In some places, he said, "it's going to be weeks at least before people can get back."

Branigin reported from Washington.