NEW ORLEANS (AP) — Gunfire and arson blazes disrupted the evacuation of 25,000 people from the Superdome on Thursday, as National Guardsmen in armored vehicles poured into New Orleans to help restore order across the increasingly lawless and desperate city.

An additional 10,000 National Guard troops from across the country were ordered into the hurricane-ravaged Gulf Coast to shore up security, rescue and relief operations in Katrina's wake. That brought the number of troops dedicated to the effort to more than 28,000, in what may be the biggest military response to a natural disaster in U.S. history.

"The truth is, a terrible tragedy like this brings out the best in most people, brings out the worst in some people," said Mississippi Gov. Haley Barbour on NBC's "Today" show. "We're trying to deal with looters as ruthlessly as we can get our hands on them."

The first of 500 busloads of people who were evacuated from the hot and stinking Louisiana Superdome arrived early Thursday at their new temporary home — another sports arena, the Houston Astrodome, 350 miles away.

But the ambulance service in charge of airlifting the sick and injured from the Superdome suspended flights after a shot was reported fired at a military helicopter. Richard Zuschlag, chief of Acadian Ambulance, said it had become too dangerous for his pilots.

The military, which was overseeing the removal of the able-bodied by buses, continued the ground evacuation without interruption, said National Guard Lt. Col. Pete Schneider. But Schneider said fires set outside the arena were making it difficult for buses to get close enough to pick people up.

President Bush urged a crackdown on the looting and other lawlessness that have spread through New Orleans.

"I think there ought to be zero tolerance of people breaking the law during an emergency such as this — whether it be looting, or price gouging at the gasoline pump, or taking advantage of charitable giving or insurance fraud," Bush said. "And I've made that clear to our attorney general. The citizens ought to be working together."

On Wednesday, Mayor Ray Nagin offered the most startling estimate yet of the magnitude of the disaster: Asked how many people died in New Orleans, he said: "Minimum, hundreds. Most likely, thousands." The death toll has already reached at least 110 in Mississippi.

If the estimate proves correct, it would make Katrina the worst natural disaster in the United States since at least the 1906 San Francisco earthquake and fire, which was blamed for anywhere from about 500 to 6,000 deaths. Katrina would also be the nation's deadliest hurricane since 1900, when a storm in Galveston, Texas, killed between 6,000 and 12,000 people.

Nagin called for a total evacuation of New Orleans, saying the city had become uninhabitable for the 50,000 to 100,000 who remained behind after the city of nearly a half-million people was ordered evacuated over the weekend, before Katrina blasted the Gulf Coast with 145-mph winds.

The mayor said that it will be two or three months before the city is functioning again and that people would not be allowed back into their homes for at least a month or two.

With New Orleans sinking deeper into desperation, Nagin also ordered virtually the entire police force to abandon search-and-rescue efforts Wednesday and stop the increasingly brazen thieves.

"They are starting to get closer to heavily populated areas — hotels, hospitals, and we're going to stop it right now," Nagin said.

In a sign of growing lawlessness, Tenet HealthCare Corp. asked authorities late Wednesday to help evacuate a fully functioning hospital in Gretna after a supply truck carrying food, water and medical supplies was held up at gunpoint.

"There are physical threats to safety from roving bands of armed individuals with weapons who are threatening the safety of the hospital," said spokesman Steven Campanini. He estimated there were 350 employees in the hospital and between 125 to 150 patients.

Tempers flared elsewhere across the devastated region. Police said a man in Hattiesburg, Miss., fatally shot his sister in the head over a bag of ice. Dozens of carjackings were reported, including a nursing home bus. One officer was shot in the head and a looter was wounded in a shootout. Both were expected to survive.

Looters used garbage cans and inflatable mattresses to float away with food, clothes, TV sets — even guns. Outside one pharmacy, thieves commandeered a forklift and used it to push up the storm shutters and break through the glass. The driver of a nursing-home bus surrendered the vehicle to thugs after being threatened.

Hundreds of people wandered up and down shattered Interstate 10 — the only major freeway leading into New Orleans from the east — pushing shopping carts, laundry racks, anything they could find to carry their belongings.

On some of the few roads that were still open, people waved at passing cars with empty water jugs, begging for relief. Hundreds of people appeared to have spent the night on a crippled highway.

The floodwaters streamed into the city's streets from two levee breaks near Lake Pontchartrain a day after New Orleans thought it had escaped catastrophic damage from Katrina. The floodwaters covered 80 percent of the city, in some areas 20 feet deep, in a reddish-brown soup of sewage, gasoline and garbage.

The Army Corps of Engineers said it planned to use heavy-duty Chinook helicopters to drop 15,000-pound bags of sand and stone into a 500-foot gap in the failed floodwall.

But the agency said it was having trouble getting the sandbags and dozens of 15-foot highway barriers to the site because the city's waterways were blocked by loose barges, boats and large debris.

The full magnitude of the disaster had been unclear for days — in part, because some areas in both coastal Mississippi and Louisiana are still unreachable, but also because authorities' first priority has been reaching the living.

In Mississippi, for example, ambulances roamed through the passable streets of devastated places such as Biloxi, Gulfport, Waveland and Bay St. Louis, in some cases speeding past corpses in hopes of saving people trapped in flooded and crumbled buildings.