Venezuela's president Nicolas Maduro moved quickly on Wednesday to swear in a new assembly with extraordinary powers as he faced charges that turnout figures for the body's election were "manipulated".
The firm that supplies Venezuela's voting technology, Smartmatic, said official figures in Sunday's election were "tampered with" in such a way that the turnout appeared greater than it was.
The election of the new super-assembly, with candidates selected from the ruling party, drew international condemnation as a power grab by the unpopular Mr Maduro, whose leftist government is beset by violent street protests and an economy on the brink of collapse.
Those fears were underscored on Tuesday when two prominent opposition leaders were hustled off to jail in the middle of the night by armed members of the Venezuelan intelligence services.
Mr Maduro planned to swear in the 545 members of the so-called Constituent Assembly, including his own wife and son, on Wednesday at a concert arena in Caracas, his press office said.
They will then take their seats on Thursday in the formal chamber of the National Assembly, which is now controlled by the opposition but whose powers will be superseded by the new body.
The opposition has called a major rally for Thursday as well.
Venezuelan officials claim that more than 40 per cent of the country's 20 million voters cast ballots on Sunday, which were boycotted by the opposition.
But in London, Smartmatic chief executive Antonio Mugica told reporters: "Based on the robustness of our system, we know, without any doubt, that the turnout of the recent election for a National Constituent Assembly was manipulated."
"We estimate the difference between the actual participation and the one announced by authorities is at least one million votes," he said.
The British company, which has worked in Venezuela since 2004, deplored the fact that opposition parties did not provide election auditors or send representatives to the tabulation centre when the results report was issued.
It noted that political parties usually received printed copies of election returns of all polling stations, to allow them to compare these printed records against the results published later by the electoral commission.
"This protocol has been followed in all Venezuelan elections since 2004, except for the elections last Sunday, because the opposition didn't participate," Mr Mugica said.
The opposition says turnout was closer to 12 per cent — on a par with the population of state employees, who were under major pressure to vote.
According to polling firm Datanalisis, more than 70 per cent of Venezuelans oppose the new assembly.
On Tuesday, Luis Emilio Rondon, the only opposition representative on the National Electoral Council's five-member board, charged there were irregularities in Sunday's elections.
"I consider to be reasonable the doubts the people may have about the results offered by the CNE," he said, referring to the election authority by its Spanish acronym.
The National Assembly on Tuesday agreed not to recognise "the fraudulent and illegitimate" Constituent Assembly, which will have the power to dissolve it.
More than 125 people have been killed in clashes between security forces and protesters since April 1 in an uptick of resistance to the Maduro government.
UN secretary general Antonio Guterres on Wednesday urged Mr Maduro's administration to "lower tensions" and "find avenues for political dialogue," an appeal echoed by European Union diplomatic chief Federica Mogherini's spokeswoman.
Ms Mogherini also said the EU refused to recognise the new Constituent Assembly.