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 hundreds of busloads of people 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 taking 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 Superdome were making it difficult for buses to get close enough to pick people up.

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.''