Venezuelan opposition leader Juan Guaido has called for a military uprising to oust President Nicolas Maduro amid mounting violence on the streets of the capital.

At least 52 people have so far been injured in clashes with police and the army, Magia Santi, president of Salud Chacao Medical Centre, told CNN.

Of the injured, he said 32 were wounded by rubber bullets, one had a gunshot wound, 16 suffered “traumatic injuries” and three had respiratory difficulties.

News footage earlier showed a National Guard armoured car slamming into protesters who were throwing stones and hitting the vehicle.

Several dozen armed troops accompanying Mr Guaido clashed with soldiers supporting the President at a rally outside the La Carlota air base in Caracas on Wednesday morning (Australian time).

Mr Guaido, in Twitter posts, wrote that he had begun the “final phase” of his campaign to topple Mr Maduro, calling on Venezuelans and the armed forces to back him ahead of May Day mass street protests planned for later Wednesday.

“The moment is now!” he wrote. “The future is ours: the people and Armed Forces united to put an end” to Mr Maduro’s time in office.

Mr Guaido took to the streets flanked by military personnel. Photo:Getty

Tens of thousands of people were marching in Caracas in support of Mr Guaido on Wednesday morning (Australian time), clashing with riot police along the main Francisco Fajardo thoroughfare.

Defence Minister Vladimir Padrino called the latest instability a “coup movement” but several hours after Mr Guaido’s announcement there was no sign of any other anti-Maduro military activity.

Venezuela’s Foreign Minister Jorge Arreaza denied there was a military coup attempt underway and accused Mr Guaido of operating under orders from Washington.

“It is not a coup attempt from the military. This is directly planned in Washington, in the Pentagon and Department of State, and by Bolton,” Arreaza told Reuters in a phone interview from Caracas, referring to US national security adviser John Bolton.

“They are leading this coup and giving orders to this man (Juan) Guaido,” he said.

US President Donald Trump said he was monitoring the situation in Venezuela very closely and reiterated US support for Mr Guaido.

“The United States stands with the People of Venezuela and their Freedom!” Mr Trump wrote on Twitter.

I am monitoring the situation in Venezuela very closely. The United States stands with the People of Venezuela and their Freedom!

— Donald J. Trump (@realDonaldTrump) April 30, 2019

Repeated opposition attempts to force Mr Maduro, a socialist, from power through massive protests and calls on the military to act have so far failed.

Mr Maduro, a former bus driver who took office after the death of political mentor President Hugo Chavez in 2013, said on Tuesday he had spoken with military leaders and that they had shown him “their total loyalty”.

“Nerves of steel!” Mr Maduro wrote on Twitter. “I call for maximum popular mobilisation to assure the victory of peace. We will win!”

The call for an uprising is Mr Guaido’s boldest effort yet to persuade the military to rise up against Mr Maduro.

If it fails, it could be seen as evidence that he lacks the support he says he holds. It might also encourage the authorities, who have already stripped him of parliamentary immunity and opened multiple investigations into him, to arrest him.

“Whatever happens now, we won’t let ourselves be stopped. Our process is moving on step by step, in accordance with our constitution. We continue to stand for nonviolence,” Mr Guaido told German international broadcaster Deutsche Welle in an interview released on Tuesday.

Mr Guaido has said Wednesday’s protests will be the largest march in Venezuela’s history and part of the “definitive phase” of his effort to take office in order to call fresh elections.

-with AAP