National Electoral Council President Tibisay Lucena announced just before midnight that turnout in Sunday's vote was 41.53 percent, or 8,089,320 people. Members of the opposition said they believed between 2 million and 3 million people voted and one well-respected independent analysis put the number at 3.6 million.
The electoral council's vote counts in the past have been seen as reliable and generally accurate, but the widely mocked announcement appeared certain to escalate the polarization and political conflict paralyzing the country.
Opposition leader Henrique Capriles, the governor of the central state of Miranda, urged Venezuelans to protest Monday. Maduro said he would use the assembly's powers to bar opposition candidates from running in gubernatorial elections in December unless they sit with his party to negotiate an end to hostilities that have generated four months of protests that have killed at least 125 and wounded nearly 2,000.
"The people have delivered the constitutional assembly," Maduro said on national television. "More than 8 million in the middle of threats ... it's when imperialism challenges us that we prove ourselves worthy of the blood of the liberators that runs through the veins of men, women, children and young people."
Several nations including Argentina, Canada, Colombia, Mexico, Panama, Paraguay, Spain, Britain and the United States said they would not recognize Sunday's vote.
"The United States condemns the elections... for the National Constituent Assembly, which is designed to replace the legitimately elected National Assembly and undermine the Venezuelan people's right to self-determination," U.S. State Department spokeswoman Heather Nauert said in a statement.
It threatened further "strong and swift" sanctions on Maduro's government.
The U.S. envoy to the United Nations, Nikki Haley, condemned the vote as a "sham" -- a word also used by Britain's junior foreign minister, Alan Duncan, and many experts.
"The vote means the end of any trace of democratic rule. Maduro's blatant power grab removes any ambiguity about whether Venezuela is a democracy," said Michael Shifter, head of the Inter-American Dialogue research center.
Latin America specialist Phil Gunson, senior analyst at Crisis Group, called the vote "the definitive break with what remains of representative democracy in Venezuela."
"It will accelerate the longer-term trend towards economic, social and political collapse unless those in a position to change course do so, and begin to negotiate a restoration of democracy and economic viability," he said.
According to polling firm Datanalisis, more than 70 percent of Venezuelans oppose the idea of the new assembly -- and 80 percent reject Maduro's leadership.
Venezuelans also protested in Miami, Madrid and various Latin American cities.
The number of Venezuelans living abroad has soared as the once-booming oil producer has descended into a devastating economic crisis marked by shortages, runaway inflation, riots and looting.