Venezuela's President Nicolas Maduro has announced the creation of a new popular assembly with the ability to rewrite the constitution, a move that opponents labelled a power grab aimed at defusing anti-government protests.

The opposition has been demanding general elections to try and end the socialists' 18-year rule.

"I don't want a civil war," Maduro told a large May Day rally of supporters in Caracas business hub, while elsewhere across the city security forces fired tear gas at youth hurling stones and petrol bombs after opposition marches were blocked.

Maduro has triggered an article of the constitution that creates a super-body known as a "constituent assembly".

It can dissolve public powers and call general elections, echoing a previous assembly created by his predecessor, Hugo Chavez, in 1999 soon after he won office.

"I convoke the original constituent power to achieve the peace needed by the Republic, defeat the fascist coup, and let the sovereign people impose peace, harmony and true national dialogue," Maduro told red-shirted supporters.

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The new constitution-writing entity would be "a citizen's constituent body, not from political parties - a people's constituent body," he said, adding that the National Electoral Council would start work on the process on Tuesday.

Only half of the 500-member assembly, or less, would be elected, he said.

Opponents fear Maduro would stuff the assembly with supporters and manipulate the elected seats by giving extra weight to pro-government workers and unions.

They said it was another attempt to sideline the current opposition-led National Assembly and potentially avoid elections amid a bruising recession and protests that have led to 29 deaths in the last month.

Call for mass protests

Julio Borges, National Assembly president, on Monday evening called on Venezuelans to rebel.

"This is a scam to deceive the Venezuelan people with a mechanism that is nothing more than a coup," Borges said, urging Venezuelans to block streets early on Tuesday and hold another march on Wednesday.

On Monday night, opposition supporters banged on empty pots and pans from their windows in a traditional Latin American form of protest.

"People, into the streets! You must disobey such lunacy!" opposition leader Henrique Capriles said on Twitter.

Since anti-Maduro unrest began in early April, more than 400 people have been injured and hundreds more arrested.

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Al Jazeera's Lucia Newman, reporting from Caracas, said protests are the main tool the opposition has to oppose the government's moves.

"The National Assembly is controlled by the opposition but it has been stripped of most of its powers. It's been unsuccessful in trying to pass laws to curtail the government's power," she said.

"So really popular pressure is the only thing they have, and possibly international pressure but that too has not really wielded many results until now. The Venezuelan government withdrew from the Organization of American States just a few days ago, the one regional organisation that was planning to really put pressure on President Maduro."

While Maduro alleges a US-backed coup plot, opponents say he has wrecked the economy and become a tyrant.

Earlier on Monday riot police officers clashed with hundreds of protesters, some throwing stones, who tried to break through security barriers to the electoral council headquarters.

National Guard troops shot tear gas in a district of west Caracas towards hundreds of opposition protesters who were gathering to march.

Elsewhere, the National Guard blocked marchers pouring towards a major highway in front of the Avila mountain which towers over Caracas' northern edge.

Opposition supporters cheered as youth ran to the front, carrying makeshift shields made from rubbish bin lids, wood and even a satellite dish.

Some, wearing motorbike helmets, swimming goggles or bandanas over their mouths, threw stones and petrol bombs at the security line, with a protester yelling, "No one turn back!"

Others blocked roads in Caracas' wealthier Chacao area with branches and fences.