Evacuation efforts had to be suspended after shots were fired at rescue boats and a military helicopter. A National Guardsman was shot. Gunfire rattled through the city. Ambulances were hijacked or tipped on their sides, and one fully functioning hospital asked to be evacuated after a supply truck carrying medical supplies was held up at gunpoint. Staff at another hospital came under sniper fire as they tried to evacuate patients.

As New Orleans was engulfed by lawlessness, President Bush dispatched 10,000 more troops to the city to try to regain order. Kathleen Blanco, the Governor of Louisiana, appealed last night for 40,000 uniformed troops. Ray Nagin, the city mayor, ordered almost his entire police force to abandon search-and-rescue duties to fight the armed gangs looting, seizing vehicles and setting fire to buildings.

The scenes at the city’s Superdome and convention centre, where tens of thousands of survivors have been trapped for days, were appalling. There was no food, water or power. Fights were breaking out. Eddie Compass, the city’s police chief, sent 88 police officers to quell the violence but they were beaten back by a mob. “We have individuals who are getting raped, individuals who are getting beaten,” Mr Compass said. Reporters on the scene described corpses lying outside the convention centre. Last night Mr Nagin issued what he described as a desperate SOS. He said: “The convention centre is unsanitary, unsafe and we are running out of supplies for 15,000 to 20,000 people.”

Lourdes Muñoz Santamaría, a Spanish politician trapped with her family in the convention centre, said: “We are getting desperate with bodies floating on the premises. I’ve seen several people die. The latest victim was a baby.”

A CNN reporter on the scene said: “There is absolutely nobody in control. There are dead bodies under blankets. Nobody is here to help. There are no National Guard. There are no police. I never imagined I would see such a scene in a major American city.”

Thomas Jessie, a roofer inside the convention centre, said: “We got dead bodies sitting next to us for days. I feel like I’m going to die. People are going to kill you for water. This is America. It is disgusting. We feel we’ve been forgotten.”

President Bush, who is to visit the stricken states of Louisiana and Mississippi today, found himself facing huge refugee and law-and-order crises. Describing the devastation wrought by Katrina as greater than that caused by the 9/11 attacks on New York, he said that there should be no leniency for the armed gangs and profiteers. “There ought to be zero tolerance of people breaking the law in an emergency such as this.”

The White House said that an additional 10,000 National Guard were being deployed to the region, bringing the total military presence on the Gulf Coast to 28,000. However, many New Orleans residents asked angrily why more military assistance was not yet on the ground, and why no food and water, or troops, had been air dropped into the city.

Mary Landrieu, a Louisiana Democrat senator, said that the hurricane had killed thousands in New Orleans alone, which would make the storm the deadliest natural disaster in the US since the 1906 San Francisco earthquake.

Numerous fires blazed across the city, many around the Superdome, making it difficult for buses to get close enough to pick people up. Armed gangs waded through the flooded streets emptying shops of televisions, jewellery and guns, with the National Guard and police nowhere to be seen.

Richard Zeuschlag, head of the air ambulance service that suspended rescue operations at the Superdome, said that his paramedics were crying for military protection because they were so scared. He said that one helicopter tried to land at a hospital, but the pilot reported seeing 100 people on the landing pad, some with guns. “He was frightened and would not land,” Mr Zeuschlag said.

Last night, search and rescue helicopters and boats defied stormy squalls over New Orleans to continue saving lives.

Some neighbourhoods under water showed few signs of life but others were dotted with pockets of humanity, some clutching seat cushions and polystyrene boxes to use as flotation aids. Others were holding umbrellas to keep the rain off as they waded chest-deep in water covered with an oily sheen of spilled petrol from thousands of floating vehicles, which in some places had caught fire.

Meanwhile, water levels finally began to drop as army engineers continued frantic efforts to plug the two massive breaches in the city’s defensive levees.

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