Attorney General Luisa Ortega, one of Maduro's biggest rivals, said she has opened an investigation into "scandalous" electoral fraud after the British technology firm contracted to handle the vote said Maduro had exaggerated the turnout.
Maduro denied the accusations of trampling on democracy in Venezuela with Sunday's controversial election for an all- powerful "Constituent Assembly," dismissing them as a "reaction by the international enemy."
The firm, Smartmatic, said in a London news conference that the official figures from the election were "tampered with" to make turnout appear greater than it was.
"We are facing an unprecedented, serious incident that represents a crime," Ortega told CNN.
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The electoral authority itself -- criticised as a Maduro mouthpiece -- denied the vote-tampering allegation as "an irresponsible contention based on estimates with no grounding in the data."
The new 545-member body -- whose members include Maduro's wife and son -- will have sweeping powers to dissolve the opposition-majority congress, pass laws and write a new constitution.
Despite months of violent protests and international condemnation, Maduro insists it is the solution to a drawn-out economic and political crisis gripping Venezuela.
"It has been proposed that the installation of the National Constituent Assembly, instead of being held tomorrow, be organized in peace and calm, with all necessary protocol, on Friday at 11:00 am (1500 GMT)," he said.
He said the reason for the delay was that 35 newly elected members had not yet been officially declared by electoral authorities.
Venezuela has been rocked by four months of clashes at anti-Maduro protests that have left more than 125 people dead.
The United States imposed direct sanctions on Maduro, calling him a "dictator," while the European Union joined the US, Mexico, Colombia and Argentina in saying it would not recognize the new assembly.