The National Guard arrived here in force Friday and moved to speed the evacuation of tens of thousands of desperate people trapped since Hurricane Katrina ravaged the Gulf Coast.

Armed with water and food -- and with M-16 rifles in clear view -- the troops deployed in the flooded streets to aid residents and control the violence that has gripped the besieged city. The convoys of troops began to provide badly needed security for the thousands of refugees gathered at the city's convention center and the Louisiana Superdome, but much of New Orleans remained under no authority.

Fires broke out sporadically throughout the day; some were left to burn. Plumes of smoke from a morning explosion and fire at a riverfront chemical depot billowed across the sky.

Amid warnings and pleas by local and state officials that people were dying by the hour, President Bush flew here and saw firsthand an airport turned partially into a large field hospital. For much of the day, the tarmac was home to the injured and infirm, brought there for triage or treatment. Some walked or staggered; others waited in wheelchairs or on stretchers.

Bush also visited Biloxi, Miss., where officials said fuel and medical personnel are running dangerously low statewide and where housing along a 50-mile stretch of coastal communities is nearly nonexistent.

Early in the day, Bush said the federal response since the hurricane landed Monday was "not acceptable," and later, in Mobile, Ala., he promised that "what is not working right, we're going to make right." His assurances were backed by congressional approval of a $10.5 billion federal relief bill, which he signed. The emergency legislation constitutes the largest undertaking of its kind in U.S. history.

Late in the afternoon, in briefings at the emergency operations center in Baton Rouge, La., Federal Emergency Management Agency official Bill Lockey acknowledged that FEMA had been caught unprepared.

"It seems our planning was inadequate," said Lockey, the agency's coordinating officer in the Louisiana disaster area. "We worked on it, we exercised for it, but the reality of it -- we've been working as hard as we can do. I've yet to be in a disaster where it went right."

In the Crescent City, the sun set again on scenes of staggering misery. New Orleans Mayor C. Ray Nagin despaired that "the people of our city are holding on by a thread. Time has run out," he said in a statement to CNN. "Can we survive another night? And who can we depend on? Only God knows."

Don Smithburg, chief executive officer of the LSU Health Sciences Center in New Orleans, which oversees eight public hospitals in the city, predicted many patients would die if not moved within the next 24 hours from grim conditions. Some hospital staff members were suffering from dehydration and had resorted to giving themselves intravenous fluids so they could continue working, he said.

At the Louisiana Superdome, still a vast bowl of filth and stench, those remaining ringed the outside perimeter and waited through another day of heat, hunger and exhaustion. Guardsmen passed out water to some there and on the streets while moving cargo from the stadium to the convention center -- even as they kept their eyes trained on rooftops and blind corners.

"We have to look out for snipers, unfortunately," said one member of the Louisiana National Guard. "That's been a big problem down here. They're like terrorists."

One military official said that as of Friday morning, 4,200 people had been evacuated, including 1,000 from the convention center, where many times that number had waited for days without aid, and where several corpses lay on the street Thursday.

A senior military officer said that commanders had not been aware of the large and desperate concentration of people at the convention center until Wednesday. He said the focus had been on evacuating the Superdome and conducting other emergency operations in the city.

"It had not perhaps been raised to our consciousness by the reports we had received," Maj. Gen. Richard Rowe, the chief operations officer for the U.S. Northern Command, said in a phone interview.

Neither state nor federal officials could offer any estimates on the number of residents still in New Orleans, where some still await rescue from rooftops.

Many of the refugees lucky enough to get out Friday were nonetheless a long way from a cot and meal. After accepting 15,000 people, Houston officials declared the Astrodome full and closed it to any more arrivals. They conceded they had overestimated the facility's capacity and began readying the Reliant Arena down the street and the convention center downtown for thousands more.

About 20 packed buses waited hours there for their passengers to be processed, even as other buses were sent four hours farther west to San Antonio.

Lockey said FEMA had rented more than 2,000 buses from across the country for the continuing evacuation and is moving them to the region. He said the agency has received offers of shelter from other states and the District and identified 2,600 available hospital beds in 12 states. The agency is working with the Department of Defense and the Veterans Administration to move patients from New Orleans into those facilities.

One state senator announced he had lined up barges to float aid down the Mississippi River to stranded individuals.

The U.S. Energy Department reported Friday night that 1.5 million customers remained without electric power because of Katrina -- down slightly from 1.8 million without power on Thursday. The commanders on the ground spoke assuredly of the progress soon to be made. Lt. Gen. Steven Blum of the National Guard said that many of the Guardsmen arriving in Louisiana had just returned from assignments overseas and are "highly proficient in the use of lethal force." He pledged to put down the violence "in a quick and efficient manner."

"The cavalry is and will continue to arrive," he said.

By Friday night, the number of National Guard forces was expected to reach 11,700 in Louisiana and 8,000 in Mississippi, with more than 5,000 troops still to come over the weekend, according to Maj. Gen. Ronald Young, the director of the Guard's Joint Staff.

In briefings at the emergency operations center in Baton Rouge, federal officials detailed the parameters of their effort. Dolph Diemont, regional emergency transportation official with the Department of Transportation, said the department had dispatched 1,400 trucks that have transported 17.1 million Meals Ready to Eat (MREs) , 5 million pounds of ice and 369 generators, as well as thousands of blankets and cots.

"We are positively making progress every day, every minute," he said.

Gen. Mark Graham, deputy commander for the Katrina task force overseeing rescue and relief, said six planes airlifted 438 people out of the city Friday. That effort is expected to retrieve many more Saturday as additional aircraft arrive, and at least one military airfield will open to increase the numbers.

U.S. Coast Guard spokeswoman Sharon Richey counted more than 2,500 Coast Guard personnel on the scene with more than 50 aircraft. Four Coast Guard cutters are stationed in the Mississippi River to offer communications and logistics support, as well as numerous other vessels.

Twenty-five foreign countries agreed to release the equivalent of 2 million barrels of oil per day from strategic fuel reserves. Several Asia-Pacific nations -- including tsunami-battered Sri Lanka -- promised to send money and disaster experts.

But officials noted the daunting challenges ahead. At least another 1,000 hospital patients were waiting to be flown out of New Orleans, about one and a half times the total already airlifted. Maj. Gen. Bruce Green, an Air Force medical officer, expressed concern about the risk of those still in the city suffering from dehydration and stress or succumbing to mosquito-borne diseases.

As reports continued of hungry and thirsty people isolated across the Gulf Coast, angry questions were being asked about why the military has not been dropping food packets for them -- as was done in Afghanistan, Bosnia and in the aftermath of the Asian tsunami.

Bill Wattenburg, a consultant for the University of California Lawrence Livermore National Laboratory and one of the designers of the earlier food drop programs, said that he has lobbied the administration and the military to immediately begin something similar. He said he was told that the military was prepared to begin, but that it was awaiting a request from FEMA.

"We know very well how to do this, and it's just incomprehensible that we're not," Wattenburg said.

At nightfall, dozens again clustered under overhangs and awnings on the sidewalks in the Superdome area, saying they were either too afraid to go in or had tried and been refused admittance.

Canaan Spriggs, 31, and his extended family, including three infants, again prepared to sleep on the floor of a nearby parking garage. He said he was pleased by the sight of the military convoys but that the city was far from tame.

"It's quiet now, but the nighttime is wild," he said. "They're sugarcoating it on the news. Come out here at night, but only if you have the National Guard with you. There are gunshots, and you hear people screaming for help.

"They'll need a lot of people out here to keep order," he warned.

Staff writers Justin Blum, Bradley Graham, Sari Horwitz, Marc Kaufman, Shankar Vedantam and Josh White in Washington, Jacqueline Salmon in Baton Rouge and Peter Whoriskey in New Orleans contributed to this report.

Hurricane survivors are evacuated from wretched conditions at the Superdome by National Guard troops and other rescue personnel. U.S. Army National Guard soldiers drive past a burning building as they ride in a convoy to distribute food and water to stranded victims in New Orleans. Victims of Hurricane Katrina wait outside the Superdome to be evacuated on Friday as troops poured into New Orleans. In 2002, the French Quarter in "The City That Care Forgot," as locals like to call New Orleans.

Refugee John Wall says he was injured in a fight in the Superdome.