A new constitutional assembly has ousted Venezuela's defiant chief prosecutor during its first day of work, deepening the political crisis in the country and signalling that President Nicolas Maduro's embattled government intends to move swiftly to try and consolidate power.

Cries of “traitor” erupted as the 545 pro-government delegates voted unanimously to remove Luisa Ortega from her post as the nation's top law official and replace her with a staunch government supporter.

Ms Ortega had become Mr Maduro's main challenger from within the ruling socialist movement since the opposition started a round of protests in April. Many of the street marches turned violent, leaving more than 120 dead as protesters clashed with police.

The new assembly – which Mr Maduro said was required to help bring peace – said it was acting in response to a ruling by the loyalist Supreme Court, which banned Ms Ortega from leaving the country and freezing her bank accounts while it weighs criminal charges against her for alleged irregularities.

Ms Ortega, a longtime government supporter who broke with the socialist government in April, refused to recognise the decision and vowed to continue defending the rights of Venezuelans from Mr Maduro's “coup” against the constitution “with my last breath”.

“This is just a tiny example of what's coming for everyone that dares to oppose this totalitarian form of government,” Ms Ortega said in the statement she signed as chief prosecutor. “If they're doing this to the chief prosecutor, imagine the helpless state all Venezuelans live in.”

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Ms Ortega had opposed the inauguration of the new assembly citing allegations of voting fraud during last weekend's election of the new body. The results have come under mounting scrutiny after the international company that provided the electronic voting machines said that “without any doubt” the official turnout had been tampered with – a charge dismissed by Mr Maduro and the National Electoral Council.

The new constitutional assembly can override the traditional National Assembly, which the opposition has been in control of since elections in 2015. The opposition boycott of the vote for the new assembly guaranteed that all candidates for the new body would be Maduro allies.

Earlier on Saturday, Ms Ortega was pushed and barred from entering her office by dozens of national guardsmen in riot gear who took control of the entrance to the building. Ms Ortega labelled the move a “siege”, tweeting photos of dozens of troops outside her office. “I denounce this arbitrary act before the national and international community,” one tweet read.

Assembly delegates later swore in as her replacement Ombudsman Tarek William Saab, who was recently sanctioned by the administration of Donald Trump for failing to protect protesters from abuses in his role as the nation's top human rights official.

Opposition protesters clash with security forces in Venezuela

Members of the all-powerful constitutional assembly had pledged in their first meeting to move quickly against Mr Maduro's opponents.

“Don't think we're going to wait weeks, months or years,” former Foreign Minister Delcy Rodriguez said on Friday after she was chosen to lead the assembly. “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.”

The constitutional assembly was seated despite strong criticism from the United States, other countries and the Venezuelan opposition, which fear the it will be a tool for imposing dictatorship.

President Juan Manuel Santos of neighbouring Colombia called the removal of Ms Ortega “the first dictatorial act” of an “illegitimate” assembly and vows solidarity with the Venezuelan people.

But Bolivian President Evo Morales was full of praise for the assembly, tweeting that “democracy is guaranteed through the votes of the people and not through bullets or hatred or violence".

Mr Maduro also wants the assembly to strip opposition lawmakers of their constitutional immunity from prosecution, saying their constant conspiring to oust him should not be protected.

While members of congress say they will only be removed by force, the opposition is struggling to regain its footing in the face of the government's strong-arm tactics and the re-emergence of old, internal divisions. Several opposition activists have been jailed in recent days, others are rumoured to be seeking exile and one leader has broken ranks from the opposition alliance to say his party will field candidates in regional elections despite widespread distrust of the electoral system.

Venezuela Supreme Court attacked with grenade thrown from police helicopter

Only a few hundred demonstrators showed up for a Friday protest against the constitutional assembly, one of the smallest turnouts in months. Those who did turn out said fear of arrest – rights groups claim there are more than 600 “political prisoners” jailed during the protests – may be keeping people at home but urged Venezuelans to remain mobilised.

“This is what the constitutional assembly will bring: more repression,” opposition legislator Miguel Pizarro said.

On Saturday, the South American trade bloc Mercosur moved to suspend Venezuela for failing to follow democratic norms.

Venezuela was previously suspended in December for failing to uphold commitments it made when it joined the group in 2012. The new decision will make it harder for the country to return to good standing since the new suspension can be lifted only when the bloc is satisfied that Venezuela has restored democratic order.

“Today in Venezuela there is no democracy,” Argentine Foreign Minister Jorge Faurie said. “Essentially what Mercosur is saying is: Without democracy, no, you cannot be a part of Mercosur.”

The constitutional assembly is made up of delegates from an array of pro-government sectors such as trade unionists, students and even representatives of Venezuelans with physical disabilities. But the agenda is expected to be set by bigger-name loyalists, including Mr Maduro's wife, son and several cabinet ministers who resigned to join the body.

It will have sweeping powers to upend institutions and in theory could even remove Mr Maduro, a fact held up by government supporters as a sign of its independence. ​