In spite having the world's largest oil reserves, Venezuela is suffering the world's highest inflation rate and chronic shortages of basic goods and medicines in an economic crisis worsened by the 2014 drop in oil prices.
Significantly, the opposition had held an unofficial referendum on July 16 in which it said 7.6 million Venezuelans voted against the new assembly - just under the level of support the government claimed to have received last Sunday.
According to the electoral authority, more than 40 percent of the 20-million-strong electorate voted.
The president of Venezuela's opposition-controlled National Assembly says the legislature will call for an investigation into claims that the official turnout figure in Sunday's election was tampered with and there was a discrepancy of at least 1 million votes.
On Wednesday, Smartmatic maintained that Electoral Council had artificially inflated the number of voters by up to one million people.
World leaders and worldwide bodies have said they will not recognize the result, including Latin American neighbors including Argentina, Peru, Chile, Colombia, Panama, Costa Rica, and Mexico, as well as the United States, Canada, and the European Union.
Maduro scoffed at the sanctions, saying he would use the assembly's powers to bar opposition candidates from running in gubernatorial elections in December unless they negotiate an end to protests. Virtually all the new body's 545 members are supporters of the leftist leader.
The opposition - a sizeable portion of the population - boycotted the vote, and an independent exit poll concluded that less than half the government's figure cast ballots.
During the vote, Venezuelans were asked to select more than 500 representatives to make up a constituent assembly. In the meantime, the Constituent Assembly has executive and legislative heft overriding any other institution. He had been convicted of instigating violence during protests against Maduro in 2014 that left 43 people dead. "It's important for the country to know the reach of this fraud and if it constitutes a crime". Its members will have to decide.
The assembly will have the power to dissolve the opposition-run congress and is expected to sack the country's chief prosecutor, who has harshly criticized Maduro this year. The opposition called for a new round of protests on Thursday.
His Attorney General, Luisa Ortega, ordered the probe to investigate "scandalous" claims that turnout in Sunday's controversial election was exaggerated.
Venezuela's government faces major challenges at present, with food shortages and soaring inflation - sparked by the US' protracted unofficial economic war against the country - being further exacerbated by a fresh wave of USA sanctions.
Critics say the assembly is meant to indefinitely extend Maduro's rule.
But in a statement late on Wednesday, China's Foreign Ministry said it had noted that the elections were "generally held smoothly", though it also noted "the reaction from all relevant sides".
That could suit Maduro, whose support is around 20 percent according to surveys by the polling firm Datanalisis.
Court officials said on Tuesday that both men had made political statements in violation of the terms of their release into house arrest, and accused them of planning to flee the country.
The political scene is deeply polarised, with little prospect so far of negotiations.