"There is absolutely no reason to stay here. There are no jobs. There are no homes to go to. No hotels to go to and there is absolutely nothing here," New Orleans Deputy Police Chief Warren Riley told a news conference on Monday.
"We advised people that this city has been destroyed and it's completely been destroyed," he said. "No food or any reason for them to stay. There is no power, trees are down, power lines are down."
Homeland Security Secretary Michael Chertoff later said he understood why people wanted to stay close to their homes - but cautioned that flood-hit areas were a severe health threat.
"As a matter of public health and matter of public safety, we have to complete this process.
The warnings came as relief workers prepared a massive operation to remove the bodies of victims of Hurricane Katrina, which slammed into the US Gulf Coast last Monday, leaving thousands feared dead and most of New Orleans under water.
The complete evacuation of New Orleans was necessary, officials said, citing the prospect of diseases caused by rotting bodies and polluted waters as well as other risks.
The death toll across the Gulf Coast was not known on Monday.
The Times-Picayune newspaper, in an open letter to President George Bush, called for the firing of every official at the Federal Emergency Management Agency, saying they failed to rescue thousands of citizens stranded by Katrina.
"We're angry, Mr. President, and we'll be angry long after our beloved city and surrounding parishes have been pumped dry," the editorial said. "Our people deserved rescuing. Many who could have been were not. That's to the government's shame."
"Every official at the Federal Emergency Management Agency should be fired, Director Michael Brown especially," the letter said. "No expense should have been spared. No excuses should have been voiced."
In neighboring Jefferson Parish, violence boiled over when 14 contractors on their way to help plug a breech in the 17th Street Canal came under fire as they traveled across a bridge under police escort, said John Hall, a spokesman for the Army Corps of Engineers.
"We're angry, Mr. President... our people deserved rescuing. Many who could have been were not. That's to the government's shame"
Police shot at eight people carrying guns, killing five or six, Deputy Police Chief W.J. Riley said.
Besides the lawlessness, civilian deaths and uncertainty about their families, New Orleans's police have had to deal with suicides in their ranks.
Two officers took their lives, including the department spokesman, Paul Accardo, who died on Saturday, according to Riley. Both shot themselves in the head, he said.
"I've got some firefighters and police officers that have been pretty much traumatized," Mayor Ray Nagin said. "And
we've already had a couple of suicides ... They need physical and psychological evaluations."
Also on Monday, European Union nations prepared aid teams, food rations, water pumps and even cruise ships to help US regions hit by Hurricane Katrina.
Countries large and small have offered aid - from tiny Luxembourg's beds and blankets to half a million food rations from Germany and Britain.
Aid and recovery expertise is
arriving from Europe
Greece put on standby two cruise ships to house refugees, and Sweden has offered aircraft to help distribute aid shipments, said Barbara Helfferich, spokesman for the executive European Commission.
US authorities made a rare request for help from Europe over the weekend, asking for anything from diapers and baby formula to forklifts and veterinarian supplies to speed up aid efforts.
Helfferich said the 25-nation EU was able to move quickly to fill requests for aid. "We have reacted very speedily and, after the aid has been requested, we have been basically able to respond in 24 hours," she said.
She said more aid pledges had come in on Sunday, including promises to ship power generators, cots, tents, first aid kits and Greece's offer of ships.
A team of 10 Belgian aid experts was leaving for Louisiana and Alabama on Monday.
The Nato alliance has also started to coordinate food aid shipments, drinking water, generators and tarpaulins through its disaster coordination center.
"I've got some firefighters and police officers that have been pretty much traumatized...and
we've already had a couple of suicides"
New Orleans mayor
The United States thanked its European allies in the 26-nation alliance for their solidarity.
Germany and Britain had already sent 570,000 emergency food rations over the weekend, while Luxembourg was preparing to send a team of five aid experts, two jeeps and 1000 camp beds and 2000 blankets.
The Netherlands had deployed its naval frigate Van Amstel, which was transporting fresh drinking water, medical supplies and much-needed helicopters.
Romania, not yet a member of the EU, was sending two teams of medical experts.
Many European countries have pledged to release parts of their strategic oil and gas reserves to stabilise global oil supplies.