CARACAS (Reuters) - President Nicolas Maduro says 8 million Venezuelans cast ballots on Sunday in an election that will give his leftist government more power, but experts say the voting was rife with irregularities and opposition leaders are crying foul.

The official vote tally is similar to those seen in victories won by late President Hugo Chavez when he was at the peak of his popularity, and tens of thousands of ecstatic red-shirted Venezuelans danced and cheered at the presidential palace when he was re-elected.

During the weekend vote to elect members of a constituent assembly, many streets were deserted and the opposition boycotted the vote championed by Maduro, who is widely blamed for an unprecedented economic meltdown marked by food shortages, runaway inflation and spiraling poverty levels.

The opposition coalition estimates only 2.5 million ballots were actually cast on Sunday, while investment bank Torino Capital said its exit polls put the number around 3.6 million.

"It's a bold-faced lie. I spent all day in Petare observing. This (8 million) number is impossible, it's total fraud," said opposition lawmaker Angel Alvarado, who told Reuters he monitored the turnout in a sprawling Caracas slum.

Furious government opponents are calling for four months of anti-government street demonstrations to be escalated. Perceptions of fraud will also embolden opposition radicals who have always claimed that voting is rigged and bolder tactics must be used to remove Maduro.

"Venezuela has to wake up, all us, we need to take to the streets," said unemployed nanny Berkys, 48, yelling to passersby to join protests on Monday as she made her way to a pharmacy hoping to find scarce medicine to treat her hypertension.

"They couldn't have won like this if we're dying of hunger," she added, asking not to be identified by her full name because she feared reprisals.

Venezuela's dissident state prosecutor, who has broken with Maduro over the erosion of democracy, also cast doubt on the turnout. And the United States on Monday imposed sanctions on Maduro after what Washington described as a "sham."

Maduro's supporters said a right-wing business elite, working in cahoots with international media, was trying to sabotage an exemplary vote to foster an "armed insurrection" against the leftist leader.

"'Chavismo' has recovered its historical voting level," said Socialist Party heavyweight Jorge Rodriguez in a televised press conference on Monday, adding some 10 million Venezuelans would have voted had the opposition not staged roadblocks on Sunday.

"When the rich lose, they shout fraud. Enough with so many lies!"

'DEMOCRACY DESTROYED'

Independent experts consulted by Reuters said a lack of oversight, and insufficient international observers, were among factors that marred Sunday's poll.

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They also cited a lack of indelible ink, to identify voters who had already cast ballots, and the fact that voters were allowed to use the polling station of their choice, opening the door to potential multi-voting.

Jennifer McCoy, a political scientist at Georgia State University who was a Venezuela election observer on several occasions, said the vote offered "very little to defend, and much to criticize."

The Atlanta-based Carter Center whose founder, former U.S. President Jimmy Carter, once said Venezuela had one of the best election processes in the world, also came out against the process.

"On top of the fact the election was illegal, the (electoral council) broke every rule in the book of electoral integrity," said Jennie K. Lincoln, the Carter Center's director for Latin America and Caribbean. "This election destroyed any vestiges of democracy that might have yet existed in Venezuela."

The constituent assembly itself has come under fire because it was designed to rewrite the constitution and possibly supplant the opposition-led congress, whose members were elected in a conventional election in December 2015.

Two dozen state workers also told Reuters they had been pressured into voting - often under threat of losing their job - if they did not support Maduro. Others said they were warned subsidized food bags would be taken away.

"I'm scared that they would have not only fired me, but also taken revenge and I can't expose my family to that," said an employee at state oil company PDVSA [PDVSA.UL], who ended up caving into pressure and voting.

"Last night I cried because I felt so helpless" she added, asking to remain anonymous because she too feared reprisals.

(Additional reporting by Mircely Guanipa in Punto Fijo, Andreina Aponte, Deisy Buitrago and Brian Ellsworth in Caracas, and Ruthy Munoz in West Palm Beach, Florida; Writing by Alexandra Ulmer; Editing by Tom Brown)