"Their mission was almost complete and we lost them so close to home. All mankind is in their debt."
George Bush added that America's space programme would continue despite the tragedy - a move which was backed by the victims' families.
The crowd, which was packed onto the lawn at the front of the space center, sang hymns to mark the lives of the astronauts.
Nose cone found
The world was shocked when the space shuttle, the Columbia, exploded as it re-entered the earth's atmosphere on Saturday.