Violence marred Venezuela's controversial election on Sunday, marked by the deaths of at least seven people, as Venezuelans trickled to the polls to vote to elect a new assembly that would rewrite the country's constitution and give the government virtually unlimited powers.

President Nicolas Maduro has pressed ahead with the vote despite the threat of further U.S. sanctions and four months of political upheaval, which has already left about 100 people dead and thousands injured and detained.

In a statement, Canada's Foreign Affairs Minister Chrystia Freeland said Ottawa "denounces and condemns" Sunday's vote and "laments the loss of life and escalating violence" that ensued.

"Despite repeated calls by member states of the Organization of American States and the international community, including Canada, to cancel the national constituent assembly, President Maduro and his government chose to take another step down the path to institutionalize authoritarian rule in Venezuela," she said.

Canada denounces & condemns significant and undemocratic action by <a href="https://twitter.com/hashtag/Venezuela?src=hash">#Venezuela</a> gov to create constituent assembly <a href="https://t.co/SYsATMAXDz">https://t.co/SYsATMAXDz</a> <a href="https://t.co/qTJrBiKWOX">pic.twitter.com/qTJrBiKWOX</a> —@CanadaFP

Offering Washington's first official response Sunday to the ballot, U.S. Ambassador to the United Nations Nikki Haley called the election a sham and said the American government will not recognize the result.

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

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

Anti-Maduro activists wearing hoods or masks faced off with security forces, who moved in quickly to disperse the demonstrators.

Authorities said seven people were killed in the unrest, though the opposition said the toll was around a dozen people, which would make Sunday one of the deadliest days since massive and sustained protests kicked off in early April.

In this photo released by Miraflores Press Office, Venezuela's President Nicolas Maduro shows his ballot after casting a vote for a constitutional assembly, Sunday, July 30, in Caracas. Maduro's opponents fear he'll use the constitutional assembly, stacked with his supporters, to replace Venezuelan democracy with a single-party authoritarian system. (Miraflores Press Office/Associated Press)

In the hours before voting began, a candidate in the election, as well as an opposition activist, were killed, officials said.

Jose Felix Pineda, a 39-year-old lawyer described as a popular candidate for the Constituent Assembly, was shot in his home in Bolivar City on Saturday night, a tweet from the country's public ministry said. A group of people broke into his residence and "fired several shots," it said.

Jose Felix Pineda, candidato popular a <a href="https://twitter.com/hashtag/AsambleaConstituyente?src=hash">#AsambleaConstituyente</a> <a href="https://twitter.com/hashtag/Venezuela?src=hash">#Venezuela</a>, asesinado hoy en su casa en Ciudad Bolívar por sicarios opositores <a href="https://t.co/PPJmWQlAix">pic.twitter.com/PPJmWQlAix</a> —@ensanro

Ricardo Campos, a youth secretary for the opposition Democratic Action party, was shot and killed during a street protest in the same city early Sunday, said Henry Ramos Allup, a deputy in the National Assembly and the party's national secretary general.

In another violent clash at about 4:30 a.m. local time, officials from the National Guard and CONAS (National Anti-Terrorism and Kidnapping Commando) raided the Caracas neighbourhood of El Paraiso, commonly known as Los Verdes (The Greens).

Tanks crashed into residential gates and armed officials ran into the courtyard, firing toward residential buildings, according to residents. Local residents said authorities also raided the neighbourhood on June 13, detaining at least 10 people and killing a dog.

President casts pre-dawn vote

Maduro asked for global acceptance on Sunday as he cast an unusual pre-dawn vote for the constitutional assembly that his opponents fear he'll use to replace his country's democracy with a single-party authoritarian system.

Accompanied by close advisers and state media, Maduro voted at 6:05 a.m. local time — far earlier and less publicly than in previous elections.

An armoured vehicle drives down a street in Caracas, Venezuela, Sunday as the Constituent Assembly election was being carried out. (Christian Veron/Reuters)

He said the "emperor" Donald Trump wanted to halt the Venezuelan people's right to vote.

"A new era of combat will begin. We're going all out with this constituent assembly," he said.

Not far from the school where Maduro voted, a 61-year-old nurse was shot and killed during a protest at a church by men accused of being pro-government paramilitaries.

Blaming the opposition

Maduro and his socialist administration deny links to violent paramilitaries and say the political opposition is responsible for the violence that has left at least 113 dead and nearly 2,000 wounded in four months of protests.

"We've stoically withstood the terrorist, criminal violence," Maduro said. "Hopefully the world will respectfully extend its arms toward our country."

A demonstrator prepares to throw back a tear gas canister during protests against the election in Caracas on Sunday. (Carlos Garcia Rawlins/Reuters)

Later in the day, Maduro's much-touted new voter identification failed to come up in the registry as he promoted the card on live national television.

The president went on state television to encourage Venezuelans to vote, admonishing them to remember to take their national identification cards and their new Patria voter ID cards. After his card was scanned, the screen read "The person does not exist or the card has been cancelled."

Turnout appears to be low

The opposition is boycotting Sunday's vote, contending the election has been structured to ensure Maduro's socialist party continues to dominate; all 5,500 candidates for the 545 seats in the constituent assembly are his supporters and the vote's success is being measured by turnout.

The government is encouraging participation with tactics that include offering social benefits like subsidized food to the poor and threatening state workers' jobs if they don't vote.

Security forces members face off against demonstrators while the Constituent Assembly election is being carried out. (Marco Bell/Reuters)

But turnout appeared to be low Sunday.

Associated Press journalists toured more than two dozen polling places in neighbourhoods across the capital and found 200 to 400 people at some, and a couple dozen at others where thousands had lined up in previous elections. Many were empty or had just a handful of voters. Some polling places in opposition-dominated neighbourhoods were closed.

In opposition-dominated eastern Caracas, riot police used tear gas to stop protesters from gathering for a march on the capital's main highway. At least three police were wounded when one of their motorcycles detonated in a powerful explosion. The cause of the explosion was not immediately clear.

'Hoping for housing'

Opinion polls say more than 70 per cent of the country is opposed to Sunday's vote. Some of those who did show up were frank about their motivations for voting: staying in the government's good graces to receive aid.

"I'm here because I'm hoping for housing," said Luisa Marquez, a 46-year-old hairdresser.

Others said they were there out of conviction that the constitutional assembly would help the government fend off what they called an international capitalist conspiracy to undermine Venezuela's socialist system with the help of the domestic opposition.

"The crisis, the shortages of food and medicine, that isn't the government's fault," said Luis Osuna, a 42-year-old private bodyguard. "Those who are attacking us to kill us with hunger and blame the government are the same enemies the government's always had."

An opposition supporter is treated for a rubber bullet wound amid clashes between protesters and security forces in Caracas Sunday. (Christian Veron/Reuters)

Once one of Latin America's wealthiest nations, Venezuela has spiraled into a devastating crisis during Maduro's four years in power, thanks to plunging oil prices and widespread corruption and mismanagement. Inflation and homicide rates are among the world's highest and widespread shortages of food and medicine have citizens dying of preventable illnesses and rooting through trash to feed themselves.

The special assembly being selected Sunday will have powers to rewrite the country's 1999 constitution but will also have powers above and beyond other state institutions, including the opposition-controlled congress.

Opposition supporters stand behind a barricade in protest against the Constituent Assembly election Sunday. (Marco Bello/Reuters)

Maduro made clear in a televised address Saturday evening that he intends to use the assembly to govern without limitation, describing the vote as "the election of a power that's above and beyond every other. It's the super power!"

He said he wants the assembly to strip opposition legislators of their constitutional immunity from prosecution and indicated he is eager to prosecute many more members of the opposition parties that control a handful of state governments along with the National Assembly, providing one of the few remaining checks on the power of the socialist party that has ruled this OPEC nation for nearly two decades.

"The right wing already has its prison cell waiting," the president said. "All the criminals will go to prison for the crimes they've committed."