The election for a new constitutional super-body underway in Venezuela under President Nicolas Maduro has been branded as a “step toward dictatorship” by the US.

US Ambassador to the UN Nikki Haley offered Washington’s first official response to Sunday’s vote in the central American nation, which has been marred by deadly protests an broadly boycotted by voters.

“Maduro’s sham election is another step toward dictatorship,” Ms Haley said in a message on Twitter. “We won’t accept an illegit govt. The Venezuelan ppl & democracy will prevail.”

President Maduro says the changes to government would begin a “new era of combat” in the crisis-stricken, oil-producing nation.

Critics say the assembly will allow President Maduro to dissolve the opposition-run Congress, delay future elections and rewrite electoral rules to prevent the socialists from being voted out of power in the once-thriving OPEC nation.

The prospect of a Cuban-style, socialist dictatorship in oil-rich Venezuela has neighbouring countries worried.

Maduro’s sham election is another step toward dictatorship. We won't accept an illegit govt. The Venezuelan ppl & democracy will prevail.

— Archive: Ambassador Nikki Haley (@AmbNikkiHaley) July 30, 2017

Protesters blocked roads and clashed with security forces as Venezuelans trickled to the polls.

President Maduro, widely disliked for overseeing an economic collapse during four years in office, pressed ahead with the vote to create the all-powerful assembly despite the threat of further US sanctions and months of opposition protests in which more than 115 people have been killed.

Three people were killed during protests on Sunday local time, the Attorney General’s office announced on Twitter. The office also announced the death of a National Guard officer.

Opposition sources put the toll much higher.

Protesters Angelo Méndez, 28, and Eduardo Olave, 39, were found with gunshot wounds inside a school in the southwestern state of Mérida, the Attorney General said.

Many schools are being used as polling centres for the National Constituent Assembly vote.

Luis Zambrano, 43, was shot in the head at a demonstration in Barquisimeto, a town about 350 kilometres west of Caracas.

A candidate to the assembly was also reportedly killed during a robbery.

Opposition parties boycotted what they called a rigged election. Their sympathisers erected barricades across roads around the country and scuffles broke out with security forces who moved in quickly to disperse hooded demonstrators.

An explosion injured a group of police officers during a protest in the capital Caracas, according to a Reuters witness, leaving eight motorbikes smouldering on a main avenue.

Watch the explosion below:

Momento en que explotó artefacto en la avenida Francisco de Miranda, #EnVIVO por: https://t.co/DTGmMHfJMM pic.twitter.com/YMjNvRewaH

— VIVOplay (@vivoplaynet) July 30, 2017

Authorities confirmed three deaths over the weekend, including the killing of a candidate to the assembly during a robbery, although opposition lawmakers said the toll was much higher.

The opposition has vowed to redouble its resistance and US President Donald Trump has promised broader economic sanctions against Venezuela after the vote, suggesting the oil-rich nation’s crisis is set to escalate.

Undeterred, President Maduro, a former bus driver and union leader narrowly elected in 2013, has accused right-wing governments of trying to sabotage 21st century Socialism.

“The ’emperor’ Donald Trump wanted to halt the Venezuelan people’s right to vote,” President Maduro said as he rapidly voted at 6 am local time in a low-income area of Caracas.

“A new era of combat will begin. We’re going all out with this constituent assembly,” he added.

With polls showing some 70 per cent of Venezuelans oppose the vote, President Maduro’s administration wants to avoid low turnout that would further undermine his legitimacy.

Venezuela’s 2.8 million state employees are under huge pressure to vote – with some two dozen sources telling Reuters they were threatened with dismissal otherwise.

– With agencies