NEW ORLEANS — A slow exodus from the smelly, sweltering Superdome began yesterday as refugees boarded giant trucks and then buses for a trip to more comfortable surroundings in the Houston Astrodome.

The evacuation was kept almost secret to avoid a stampede. People were taken a few at a time through a garage, then up a ramp to trucks that plowed through 4 feet of water and took them to the buses.

People with physical problems and the elderly were evacuated yesterday morning, loaded into a variety of vehicles and taken to hospitals. The evacuation of the more able-bodied began later.

The Federal Emergency Management Agency (FEMA) has provided about 500 buses to transport the refugees. Mayor Ray Nagin said military cargo planes were en route as a backup just in case the bus system was inadequate or the roads too flooded.

It was not immediately clear how long the refugees will be in Houston, 350 miles from New Orleans. "We're buying time until we can figure something out," said William Lokey, chief coordinator for FEMA.

Houston's government, schools, churches, hotels, hospitals and dozens of relief agencies organized to accommodate the refugees inside the Astrodome, which has been used only sporadically since the Houston Astros left six years ago for more modern quarters downtown. The facility now is more commonly used for events such as conventions, concerts and high-school football games.

Across the sprawling city, the country's fourth-largest, people rose to the sudden challenge before them. A local CBS-television affiliate held a telethon to raise money for the refugees, local hospitals began evacuating critically ill patients from Louisiana and Mississippi, and Radio Saigon Houston, a local station that broadcasts in Vietnamese, announced that Vietnamese speakers would take in hundreds of Vietnamese storm victims.

Cots and blankets for up to 25,000 people were being set up on the Astrodome's floor. A catering company set up makeshift kitchens. And the air conditioning, missing in New Orleans, was turned on.

Organizers plan to use Astrodome kitchens and locker rooms to keep refugees fed and clean, but they said they realize it won't be easy because the arena was not built to handle so many people.

"We're essentially picking up a small city and inserting it into Houston," said Frank Michel, the city's communications director. Once the storm victims arrive, officials said, the challenges will be huge: identifying the sick or emotionally traumatized, helping cash checks and find ATM machines and phones to contact their families.

There will be tight security around the Astrodome, but no curfews, Michel said.

The bus arrivals represent a second wave of storm victims in Houston. Thousands of families drove to the city seeking shelter in the days before Katrina hit, staying with friends and family or in local hotels, which are now almost all booked.

The Astrodome's schedule has been cleared through December for housing evacuees, said Kathy Walt, a spokeswoman for Texas Gov. Rick Perry.

The situation at the Superdome was quickly worsening: The air conditioning was not working, toilets were not operational, and tempers were rising.

"I'm ready to get away from here. Anybody in their right mind would be," said David Ellis, a construction worker who was on the ramp outside the Superdome, escaping the oppressive conditions.