The US military late on Sunday night issued a statement signed by Al-Zubaidi admitting he was not the mayor of Baghdad and pledging to work with the US-led administration in Iraq.
Occupation US troops arrested Al-Zubaidi on 27 April for exercising authority he did not have after he proclaimed himself mayor.
In the chaos that followed the toppling of Saddam Hussein in the US-led invasion of Iraq, Al-Zubaidi set up 22 committees to act as ministries prompting widespread confusion about who was actually in charge of Baghdad.
Al-Zubaidi said he had been elected by people representing a cross section of the Iraqi society -- although he did not clarify how or when the vote happened.
Offer to co-operate
US Central Command said on Sunday that Al-Zubaidi had issued a statement before his release pledging to work with the "coalition authority" towards rebuilding Iraq.
It quoted him as saying: "I am not the mayor of Baghdad, nor am I interested in working independently of the coalition to achieve what we all understand to be the same goal - peace and prosperity for all Iraqis."
Lt. Gen. David McKiernan, head of US land forces in Iraq, said "There will be officials elected and appointed who will represent Iraqis. I hope that, like we did with Al-Zubaidi, we can work with those Iraqis who want a better future for Iraq."