Venezuela's President Nicolas Maduro has sworn in a controversial new Constituent Assembly that has the power to rewrite the constitution, despite massive protests and state prosecutors vowing to block it.

The 545-member assembly, including Maduro's wife and son, held its inaugural session under tight security in the capital Caracas on Friday.

Delcy Rodriguez, Venezuela's former foreign minister, was nominated as president of the disputed body, which was elected on Sunday in a vote boycotted by the opposition and allegedly marred by voter fraud.

The assembly is scheduled to meet again on Saturday, and Rodriguez pledged it would be taking action against Maduro's political opponents.

"Don't think we're going to wait weeks, months or years," she said. "Tomorrow we start to act. The violent fascists, those who wage economic war on the people, those who wage psychological war, justice is coming for you."

Al Jazeera's John Holman, reporting from Caracas, said: "The superbody will hold a lot of power over not just the opposition, but 40 countries around the world are worried about the democratic consequences on Venezuela of having this superpower of the Constituent Assembly."

Earlier, the Vatican urged the Venezuelan government to "avoid or suspend" its new legislative superbody.

READ MORE: What is a National Constituent Assembly?

In a statement issued on Friday, the Vatican said, "initiatives such as the new Constituent Assembly instead of fostering reconciliation and peace, foment a climate of tension".

The Vatican also called on the security forces to refrain from using "excessive and disproportionate force" amid protests.

Hours before the inauguration, Venezuela's intelligence services transferred prominent opposition leader Antonio Ledezma from jail back to house arrest, his family said.

On Tuesday, security forces forcibly entered Ledezma's apartment before dawn and took him to a military prison.

New stage in politics

The Constituent Assembly marks a new stage in Venezuela's rule.

With unlimited powers to dissolve the National Assembly or amend laws, the new body is tasked with rewriting the 1999 constitution brought in under Maduro's late mentor and predecessor, Hugo Chavez.

Maduro claims the revised charter will pull Venezuela out of its political and economic crisis

READ MORE: Venezuela's crisis explained from the beginning

The body has faced a backlash on several fronts.

Backing opposition allegations of fraud, Smartmatic, a British-based company involved in the voting technology behind the election last Sunday, said the official turnout figure had been tampered with and exaggerated by at least a million voters.

Although brushed off by Maduro as part of a plot by "the international enemy," Luisa Ortega, Venezuela's attorney general and one of Maduro's most vocal critics, ordered an investigation.

She said prosecutors had lodged court cases seeking to have the Constituent Assembly annulled.

Venezuela has experienced four months of violent protests that have left more than 125 people dead as opposition demonstrators armed with stones and Molotov cocktails battle security forces and armed motorcycle gangs of Maduro supporters.

Sunday's vote brought the crisis to a boiling point, drawing international condemnation.

The United States has imposed direct sanctions on Maduro, calling him a "dictator," while the European Union has joined the US, Mexico, Colombia and Argentina in saying it would not recognise the new assembly.

Al Jazeera's Holman said that the ranks of protesters seem to have thinned compared with the massive turnouts two months ago.

"People here are not willing to give up despite fear and tiredness after more than 100 days of protests," he said.