Madagascar's Andry Rajoelina, who ousted President Marc Ravalomanana this week, has been installed formally as leader of the Indian Ocean island.
Tens of thousands of his supporters attended the ceremony at a sports arena in the capital, Antananarivo, but it was boycotted by many diplomats.
On Friday, the US cut off non-humanitarian aid to Madagascar and the African Union suspended its membership.
Madagascar's highest court this week approved the handover of power.
"The ambassadors to the US, France, Germany and the European Union have told us they won't be attending," an aide to the deposed Mr Ravalomanana told Reuters new agency before Saturday's ceremony.
About 2,000 of the former president's supporters reportedly held a counter-rally at Antananarivo's Democracy Square.
The BBC's Christina Corbett in Antananarivo says widespread condemnation of Mr Rajoelina's military-backed rise to power has not deterred him from throwing a lavish inauguration ceremony.
Aides close to Mr Rajoelina say they are not concerned by the string of international denouncements that has followed the former president's removal.
Mr Rajoelina, a 34-year-old ex-disc jockey, has suspended parliament and set up two transitional bodies to run the Indian Ocean island.
Questions over legality
Amid questions over the legality of his rise to power, Africa's youngest and newest leader is calling himself "president of the transitional authority", while his government reportedly called Saturday's ceremony an "installation", rather than a "swearing-in".
The former Antananarivo mayor, who has never stood for national office, has promised elections within 18 to 24 months, but foreign powers have called for polls sooner.
Mr Rajoelina wants to change the constitution, which at presents bars him from contesting presidential elections, as he is six years too young, although the Constitutional Court has already endorsed him as national leader.
Washington called the takeover a "coup" while Norway also cut aid to Madagascar, where 70% of government spending comes from overseas funds.
The EU has added its voice to the chorus of condemnation and the Southern African Development Community (Sadc) has threatened sanctions against Madagascar.
Roindefo Monja, prime minister in Mr Rajoelina's transitional administration, said on Friday the new government stood by its actions.
"The people demanded liberty and the military rallied to the popular movement, but it did not seize power... We are confident the international community will understand," he said.
There is still no word on the whereabouts of Mr Ravalomanana, whose re-election to a second term in 2006 could not save him from being ousted.
He quit after weeks of deadly street protests amid the power struggle and handed power to the military, which then named his bitter enemy Mr Rajoelina as leader.