Caracas (AFP) - Venezuela's President Nicolas Maduro formally launched moves to rewrite the constitution on Tuesday, defying opponents who accuse him of clinging to power in a political crisis that has prompted deadly unrest.

At an open air rally before thousands of supporters wearing red t-shirts, Maduro signed a document formally establishing the terms for electing members of a "constituent assembly" that will be tasked with drafting a new constitution.

His backers waved red, yellow and blue national flags in defiance of angry opposition protests after the death toll from weeks of clashes reached 53.

Elections to the new assembly will take place at the end of July, the National Electoral Council president Tibisay Lucena later said in a televised statement.

He also said gubernatorial elections pending since last December would take place on December 10 this year.

The opposition-controlled National Assembly promptly rejected Maduro's plan.

"The Venezuelan people do not want a fraudulent Madurist constitution and we will not accept it," opposition leader Henrique Capriles tweeted.

Violence has appeared to intensify in recent days with daily opposition marches calling for early elections.

Crowds doused a man in fuel and set him alight at a demonstration on Sunday. Another three people died from gunshot wounds late Monday in riots in the western state of Barinas.

The government and opposition accuse each other of sending armed groups to sow violence during demonstrations.

- Constitutional maneuvers -

Maduro has accused the opposition of "terrorism," resisting its calls for a vote on removing him from power.

The opposition blames him for an economic crisis that has caused major shortages of food, medicine and other basic goods amid soaring inflation.

It says he plans to pack the "constituent assembly" with his supporters.

Maduro said 176 of the assembly's members will be drawn from certain civil groups, which the opposition say are traditionally loyal to him and his late predecessor Hugo Chavez.

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In a move set to further inflame the opposition, Maduro said the constitutional body would sit in the chamber of the National Assembly legislature -- the only state institution nominally controlled by his opponents.

"What Nicolas Maduro has announced is nothing but a continuation of a coup against the constitution," said the opposition speaker of the legislature, Julio Borges.

"This National Assembly calls on Venezuelans to continue peaceful protests morning, noon and night in all corners of Venezuela until the constitution is respected."

Electoral specialist Eugenio Martinez said on Twitter that the constitutional assembly looked likely to be set up via "a made-to-measure election, a process designed to make the Chavista vote worth more than the opposition one."

- Death toll hits 53 -

A total of 53 people have died during demonstrations, many of them shot, public prosecutors said on Tuesday. Looting has broken out in various cities.

The man set alight on Sunday had been accused of thieving. Maduro said he was targeted for being a government supporter.

Fresh riots broke out on Monday evening in the capital and Barinas.

"Venezuelans awake today in mourning for those murdered in our beloved Barinas. Such viciousness against our people!" Capriles wrote on Twitter.

Opposition lawmakers called a session for Tuesday to discuss the violence.

- Currency controls -

The government also unveiled its latest foreign-exchange auction mechanism to boost its control of the currency on Tuesday.

The system will provide dollars within a price range determined by the central bank, Economy and Finance Minister Ramon Lobo told a news conference, without specifying the range.

Maduro has said the government is targeting the black market rate, which can reach as high as eight times above the top official rate. He blames it on a private sector "economic war."

But analysts say the rate actually reflects the collapse in prices of oil, Venezuela's main export. They also blame the government's currency controls of crippling the private sector and helping to cause shortages.

- Maduro resists -

Elected in 2013, Maduro has resisted opposition efforts to remove him since January 2016.

He has said presidential elections will take place as scheduled next year, but not before.

Despite the mounting outrage among Venezuelans, Maduro retains the military's public backing along with control of most state institutions.

He says the crisis is the result of an international conspiracy backed by the United States.