Speaking after his resignation, Michael Brown said: "As I told the President, it is important that I leave now to avoid further distraction from the ongoing mission."
Mr Bush has called in David Paulison, a veteran firefighter who had run Fema's preparedness division, to take over the federal relief effort. Mr Paulison was the official who urged Americans to stock up on duct tape and plastic sheeting in 2003 to protect against a biological or chemical attack, a recommendation widely ridiculed.
Mr Brown was pulled out of operations on Friday and replaced on the ground by Vice-Admiral Thad Allen of the US Coast Guard.
Mr Bush had praised Mr Brown for doing a "heck of a job" in the first days of the disaster, but was under pressure from Democrats and others to fire Mr Brown. The President also said he would name more people to senior positions at the agency in the coming days.
* Utility workers connected the wrong wires and caused a blackout across major parts of Los Angeles yesterday afternoon, trapping people in elevators and snarling traffic at intersections. The Police Department went on "full tactical alert," but power was restored within hours.
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- Stephen Carter