President George W Bush has promised a better response to future hurricanes after admitting to failures in the government's handling of Katrina.
Mr Bush has been leading remembrance in New Orleans a year after the storm devastated the city, on what was declared a national day of remembrance.
"If another hurricane comes there will be a better response," he said.
Nearly 1,700 people died when the storm struck Mississippi and Louisiana, and thousands became homeless.
Mr Bush said the hurricane had brought out the best in America, but there had also been shocking sights.
"Unfortunately, the hurricane also brought terrible scenes that we never thought we'd see - citizens drowning in their attics... a breakdown of law and order, and a government at all levels that fell short of its responsibilities."
The president said he took full responsibility for the federal response, and they were "addressing that which went wrong".
"The people of the Gulf Coast can know that at the federal level, and at the state level, and at the local level... we're better prepared."
The president began the day by meeting Mayor Ray Nagin for breakfast at Betsy's Pancake House.
As the president walked into the restaurant, waitress Joyce Labruzzo asked: "Mr President, are you going to turn your back on me?"
"No ma'am, not again," he replied to laughter.
Mr Bush went on to Saint Louis Cathedral to attend a service in the church left virtually untouched by Katrina.
Bells tolled in the city at 0938 (1438 GMT) to mark the moment one of the city's flood walls breached - and water engulfed the northern edges of New Orleans.
Survivors of the storm gathered outside City Hall, where Mayor Nagin said the anniversary was a difficult day for everyone.
"Trust me. We will get through it. We will get through it together," he said.
Across the city residents marched in procession and laid wreaths in still-devastated neighbourhoods, and threw them into New Orleans' canal network, to remember those killed by the flood and its aftermath.
Many of the processions were accompanied by jazz brass bands playing New Orleans' traditional funeral dirges.
One year on, many of the areas devastated by the floods have yet to be rebuilt and re-populated.
Only about 200,000 of the original near-half-million inhabitants have returned.
Mr Bush, addressing local people at a school gym, predicted that one day New Orleans would be "louder, brasher and better" than ever before.
He also urged those who have left to return. "I know you love New Orleans. And New Orleans needs you. She needs people coming home," he said.
Critics say many of those promised federal and state assistance to rebuild their homes are still waiting to receive it.
And with many of the city's former residents now dispersed across the rest of the US, there are fears that New Orleans will never regain its former character.
Mr Nagin hopes population levels will recover, even if it takes five years.
He has also pledged to defend historically black neighbourhoods - some of those worst-hit - from destruction.
More than half of the city's pre-Katrina residents were African-Americans, but many of those who have stayed live in the wealthier white areas.