A hard-left guerrilla group operating in the mountains that border Venezuela and Colombia has vowed to respond to any military intervention by the US to topple Nicolás Maduro.

In a rare interview from exile in Cuba, leaders of Colombia's National Liberation Army (ELN) said they were drawing up a strategy to help Venezuela repel any possible invasion.

The Marxist group's comments demonstrate the prospects of a protracted conflict if the US chooses to use force against Maduro, the Venezuelan President. Maduro warned Donald Trump last week that the US President was walking into his own Vietnam.

John Bolton, the US national security adviser insisted that "all options are on the table" after he was pictured holding a notepad ahead of a briefing on the Venezuela crisis that said "5,000 troops to Colombia".

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The ELN has been blamed for a wave of terror attacks in Colombia where it filled a power vacuum left by the Farc (Revolutionary Armed Forces of Colombia) when a peace deal brought the world's longest running civil conflict to a close.

Israel Ramírez Pineda, the ELN commander otherwise known as Pablo Beltrán, said that the leadership has been contemplating a strategy to help repel any ground assault. "It's certain that the US attack plan would mean that we're the first in line. We don't doubt that," he said. "If they come for us, we will respond. We won't run away and hide."

The rebels have been blamed for growing insecurity along the border with Colombia, where they are increasingly involved in illegal gold mining and other illicit trade.

Beltrán accused the Americans of staging a coup in Venezuela and said he believed the Colombian military could use the crisis to step up operations against the guerrillas with the support of Trump.

Yesterday, massive numbers of anti-government protesters rallied in a dramatic show of resolve, escalating the campaign to drive Maduro from power even as the first major defection occurred among his crucial military backers.

The demonstrations alone were unlikely to cause Maduro to buckle.

An anti-government protester with Venezuelan flag glasses, left, while supporters listen to Nicolas Maduro at another rally, above. Photo / AP
An anti-government protester with Venezuelan flag glasses, left, while supporters listen to Nicolas Maduro at another rally, above. Photo / AP

But the open defiance on the streets seeks to keep the pressure on Maduro at home while sending a message to other nations considering throwing their support behind the opposition and its leader, Juan Guaidó.

Guaidó — who heads the opposition-controlled National Assembly that last month proclaimed Maduro a usurper — addressed crowds near the European Union offices in Caracas in hopes that France, Britain and other European powers would join the United States and others to recognise Guaidó as Venezuela's de facto leader.

Maduro was already hit by a blow from within: In a video circulating on social media, an acting Venezuelan air force general, Francisco Yanez, denounced the "dictatorship" of Maduro and recognised opposition leader Guaidó's claim as the nation's legitimate interim leader.

Maduro also brought supporters out for rallies marking 20 years of socialist rule. But those crowds were vastly overwhelmed by the turnout at opposition marches in Caracas and more than two dozen cities nationwide, which appeared to be in the hundreds of thousands.

- additional reporting Washington Post