He singled out Mr Galbraith - the then deputy head of the UN mission in Afghanistan - of feeding details to the international media in an attempt to blacken his name.
"This wasn't fraud by Afghans but the fraud of foreigners, the fraud of Galbraith, or [head of the EU's observers Philippe] Morillon, and the votes of the Afghan nation were in the control of an embassy."
Mr Galbraith said the suggestion that the UN would organise electoral fraud was "absurd".
"At first I thought it was an April Fool's joke but I realised I don't have that kind of warm, personal relationship with President Karzai that he would do that," he said.
He told the BBC the suggestion that the UN had stolen votes served as a sharp reminder of the fraud that dogged the election.
Mr Galbraith, a former US diplomat, was dismissed last year after alleging that the UN was not doing enough to combat fraud in the election.
Mr Karzai is currently locked in a power struggle with parliament over his attempt to appoint all the monitors in Afghanistan's election process.
Hamid Karzai spoke of a ''massive fraud'' committed by ''foreigners''
He was not declared the clear winner in the first round of the 2009 presidential vote, but emerged as victor after the challenger in the second round stood down, following several months of argument.
Irregularities uncovered by the independent Electoral Complaints Commission included polling where the turnout was more than 100%, and others where votes were counted from places known to have been closed on election day.
Mr Karzai's comments come a day after the Afghan parliament rejected his attempt to have an all-Afghan body monitoring elections.
The BBC's David Loyn in Kabul says they show that the president will try to force this all the way through if he can, seeking to take control of the election process.
Our correspondent says it puts him on a collision course with US President Barack Obama, who has been putting pressure on the Afghan president to change his ways and crack down on corruption.
The UN in Kabul has not commented on Mr Karzai's claims.
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