The deaths, mostly from gunshot wounds, were recorded in the capital and across the country, along with reports of looting in Brazilian border state Bolivar, the Venezuelan Observatory of Social Conflict reported.
The intensity of the protest can be seen in videos posted on social media.
13 dead in two days of Venezuela unrest
Unrest started when 27 soldiers staged protest against Maduro
Unrest first kicked off on Monday when 27 soldiers rose up against Maduro at a command post in Caracas, calling upon people to "take to the streets" in a video circulated on social media.
But the most fierce began Tuesday night in capital city of Caracas and Bolivar which continued throughout yesterday, a day on which huge opposition-backed demonstrations were held against Maduro.
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Protesters pelted stones in response to military's tear gas, rubber-bullets
In Bolivar's San Felix, demonstrators burned a statue of ex-president Hugo Chavez late Tuesday night.
Chaos erupted when dozens of young people, some hooded, blocked a key route through the wealthy Altamira neighborhood, only for military personnel to fire tear gas and rubber bullets, which were, in turn, met with stones from the protesters.
Apart from Caracas, protester-deaths were reported in Tachira, Barinas, etc.
Opposition leader Juan Guaido announces himself as interim President
Yesterday, anti-riot police confronted protesters in east Caracas following the march called by opposition leader Juan Guaido, who declared himself Venezuela's interim president as part of his aim to remove Maduro. Guaido was quickly recognized by US, Canada, Brazil, and Colombia as the interim president.
Guaido plans to remove Maduro, set up transitional-government, hold elections
Yesterday's demonstration, meanwhile, marked the first significant clash between the government and the opposition in the streets after the violent protests between April and July 2017 that left 125 people dead.
Guaido, the 35-year-old opposition-led legislature head who declared himself "acting president", called for the protests two weeks ago.
His aim is to remove Maduro, set up a transitional government and hold elections.
Maduro asks US diplomats to leave Venezuela within 72 hours
Guaido accused Maduro of being a usurper following his disputed re-election in a poll dismissed as a fraud by the European Union, the US and Organizations of American States (OAS).
After Guaido was recognized by the US President Donald Trump as the interim President, Maduro broke off diplomatic ties with the US and gave the US Diplomats 72 hours to leave Venezuela.
Do you want a puppet government controlled by Washington: Maduro
In response, Guaidó urged foreign embassies to keep their diplomats in Venezuela, which would be abided by, said US secretary of state, Mike Pompeo, adding Maduro's order to withdraw its diplomats would be ignored.
Meanwhile, Maduro, in his address to his supporters gathered outside Miraflores presidential palace, said, "They intend to govern Venezuela from Washington. Do you want a puppet government controlled by Washington?"