LONDON (AFP) - The legitimacy of a powerful new assembly in Venezuela being sworn in on Wednesday was thrown further into question when the voting technology firm involved in its election said the turnout figure was "manipulated."

President Nicolas Maduro was to inaugurate the Constituent Assembly - whose loyalist 545 members include his wife and son - in a ceremony in a Caracas concert arena, his press office said.

The body, which was elected despite months of street demonstrations and international condemnation, is tasked with rewriting the constitution of the crisis-hit South American country.

It has wide powers superseding those of the opposition-controlled legislature, the National Assembly, whose palace it was to take over from Thursday, when it starts work.

The election of the new assembly on Sunday was marred by several deaths and clashes between security forces and protesters. Several Latin American countries said the vote destroyed Venezuela's democratic system.

The United States imposed direct sanctions on Maduro, whom it called a "dictator." The European Union on Wednesday joined the US, Mexico, Colombia and Argentina in saying it would not recognize the new assembly.

The British firm that supplied Venezuela's voting technology, Smartmatic, said in a London news conference on Wednesday that the official figures from the election were "tampered with" to make turnout appear greater than it was.

Venezuela's electoral authority denied that assertion as "irresponsible."

'AT LEAST ONE MILLION VOTES'

Smartmatic chief executive Antonio Mugica said: "Based on the robustness of our system, we know, without any doubt, that the turnout of the recent election for a National Constituent Assembly was manipulated."

"We estimate the difference between the actual participation and the one announced by authorities is at least one million votes," he said.

Venezuela's pro-government electoral authority had claimed more than eight million voters had taken part - 40 per cent of the 20-million-strong electorate.

The head of the National Electoral Council, Tibisay Lucena, said Smartmatic's assertion was "an irresponsible contention based on estimates with no grounding in the data."

It was an "opinion from a firm whose only role in the electoral process is to provide certain services and technical support that had no bearing on the results," she added.

The opposition has insisted turnout was closer to 3.5 million, mostly made up of state employees fearful for their jobs.

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.

National Assembly leader Julio Borges called the accusation of polling manipulation "an earthquake on a global scale."

According to polling firm Datanalisis, more than 70 per cent of Venezuelans oppose the new assembly.

Because of an opposition boycott, it was a foregone conclusion that followers and allies of Maduro's Socialist Party would fill all the Constituent Assembly posts. So the turnout figure was the key gauge of public support.

SECURITY STATE

The leftist president moved swiftly to consolidate his authority after the election.

Two prominent opposition leaders were hustled off to jail in the middle of the night by armed members of the Venezuelan intelligence services.

Security forces and pro-government motorbike gangs were active in confronting public signs of dissent.

The opposition, a coalition of disparate parties with no clear overall leader, has become increasingly sidelined and despondent.

Yet it is trying to maintain its campaign of protests, with another mass demonstration scheduled for Thursday, when the Constituent Assembly takes its seats.

Some young opposition supporters are opting for more radical action, including throwing Molotov cocktails, in what they call a "resistance" movement.

More than 125 people have already been killed in clashes between security forces and protesters since April 1, when demonstrations against Maduro intensified.

The opposition lawmakers in the National Assembly said Tuesday they will not recognize "the fraudulent and illegitimate" Constituent Assembly.

But Delcy Rodriguez, a former foreign minister who is now part of the new body, said the Constituent Assembly will kick the lawmakers out of the legislative palace, "from which we will never leave." .

DIALOGUE URGED

UN Secretary General Antonio Guterres urged Maduro's administration to "lower tensions" and "find avenues for political dialogue."

EU diplomatic chief Federica Mogherini said Wednesday the 28-nation European bloc was "ready to gradually step up" action on Venezuela if the situation deteriorates further. Some EU nations and officials were urging sanctions, but that step had not yet been taken.

Maduro has dismissed the criticism levelled at him. After the US sanctions were announced, he retorted that he will not heed "imperial orders."