Madagascar's highest court has backed the handover of power to interim leader Andry Rajoelina.
It comes a day after military leaders conferred powers to the 34-year-old when President Marc Ravalomanana quit.
But under the current constitution, the former disc jockey is six years too young to be elected president.
Zambia called for the immediate suspension of Madagascar from the African Union (AU) and Southern African Development Community (Sadc).
Zambian Foreign Minister Kabinga Pande also told a news conference in Lusaka: "Zambia rejects the unconstitutional change of government in Madagascar."
Meanwhile, France's foreign ministry said the 24-month deadline Mr Rajoelina has pledged for holding fresh elections was "too long".
A Quai d'Orsay spokesman stressed France would continue its aid to the island and "maintain its policy of co-operation".
'Back to normal'
Mr Rajoelina addressed supporters in the capital on Wednesday to promise a return to "normal life, to security and above all national reconciliation", reported Reuters news agency.
President Marc Ravalomanana 's whereabouts are currently unknown
The constitutional court announced its approval of the power transition in a radio address, after being approached by Mr Rajoelina's supporters.
The BBC's Jonah Fisher, in the capital Antananarivo, says the legal decision is part of a process of legitimising Mr Rajoelina as president after he effectively seized power with the backing of his supporters and the military.
Our correspondent says there is relief on the streets that the Indian Ocean island's power struggle, which has left more than 100 people dead since January, has ended without further bloodshed so far.
'No power vacuum'
According to a legal document obtained by AFP news agency, the constitutional court "proclaims that Mr Andry Rajoelina exercises the attributions of the president of the Republic as stated by the provisions of the constitution".
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