Protesters in Venezuela have lobbed bottles and bags of faeces at soldiers who fought back with tear gas to block the latest march in more than a month of nationwide protests against President Nicolas Maduro.

The so-called "Shit March" was held on Wednesday on the main highway through Caracas came as thousands of opposition supporters again poured onto the streets decrying Venezuela's economic crisis and demanding elections.

"These kids live in a dictatorship, they have no other option but to protest however they see fit," said Maria Montilla, 49, behind lines of youths with masks, slingshots and makeshift wooden shields.

Many carried stones and so-called "Poopootov cocktails" - faeces stuffed into small glass bottles - which they threw when National Guard troops blocked their path, firing gas and turning water cannons on the crowds.

"There's nothing explosive here. It's our way of saying 'get lost Maduro, you're useless!'," said one young protester, who asked not to be named, between tossing bottles of faeces.

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Messages giving step-by-step instructions and advice on putting together the Poopootov cocktails went viral on Venezuelan WhatsApp groups before the protest.

Some opposition sympathisers were appalled at the use of faeces, both animal and human, calling it an unsanitary and inappropriate tactic even in the face of a government they despise.

They noted that throwing faeces could increase cases of infectious diseases which are soaring due to the lack of medicine as well as basic cleaning materials such as soaps and disinfectant.

Two deaths

Wednesday's chaos in Caracas claimed another life, with 27-year-old Miguel Castillo killed during the protests, the state prosecutor's office said without giving more details.

With trouble flaring in various parts of Venezuela, there was also another death in the Andean city of Merida.

Motorbike taxi driver Anderson Dugarte, 32, died on Wednesday after being injured in a protest, the state prosecutor's office said.

Local media said he was shot in the head.

The unrest has killed at least 39 people since early April, including protesters, government sympathisers, bystanders, and security forces. Hundreds have also been hurt and arrested.

Maduro says foes are seeking a coup with US encouragement.

The opposition, which enjoys majority support after years in the shadow of the ruling Socialist Party, says authorities are denying a solution to Venezuela's crisis by thwarting a referendum, delaying local elections and refusing to bring forward the 2018 presidential vote.

They are seeking to vary tactics to keep momentum going and supporters energised.

The government accused the opposition of breaking international treaties on biological and chemical weapons by throwing faeces.

Maduro is seeking to create a new super body called a "constituent assembly," with authority to rewrite the constitution and shake up public powers. Foes dismiss it as an attempt to keep the socialists in power by establishing a biased new assembly.

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In downtown Caracas, government supporters also rallied, dancing salsa and waving pictures of Maduro's predecessor Hugo Chavez who remains venerated by many, especially the poor.

"I'm here to support the constituent process, which brings opportunities to resolve the crisis," said agriculture worker Ilian Leon, 40. "We're faithful to Chavez's legacy."

The protests so far have failed to garner massive support from poorer, traditionally pro-Chavez sectors of Venezuela's 30 million people.

But a bigger cross-section of society has been apparent at recent marches, some of which drew hundreds of thousands.

Looting has been breaking out in some cities, especially at night.

Chavez's former spy-master Miguel Torres has broken with Maduro's government, despite having served as interior minister and fighting against protests in 2014. He warned on Wednesday that the violence in Venezuela may be getting out of control.

"What is happening may be the starting point for a huge armed confrontation between Venezuelans," he told Reuters.

"Nobody wants that."