Venezuelan President Nicolas Maduro's opponents took to the streets Thursday vowing a new day of huge protests, upping the ante after deadly clashes in the beleaguered oil-rich nation.

Protesters determined to oust the leftist leader massed from the morning hours in Caracas, refusing to flinch after the most violent day yet in three weeks of unrest that have left eight people dead.

Riot police fired tear gas to break up scores of protesters on the capital's west side who were trying to join a larger march -- though there was no immediate repeat of Wednesday's violent clashes, in which three people were killed.

There was a heavy security presence in Caracas and other flashpoint cities, with armored trucks and riot police guarding the main roads.

"No surrender. It is our duty to defend the constitution," said senior opposition leader Henrique Capriles.

But some experts warned the opposition was pursuing a risky strategy.

"The stronger the protests are, the stronger the repression will be," said analyst Diego Moya-Ocampos of London-based consultancy IHS Markit Country Risk.

Numerous businesses and universities remained closed.

The center-right opposition called for employees and students to be given leave to attend the new marches. Wednesday's turnout was greatly boosted by the fact it was a national holiday in Venezuela.

Hundreds of thousands of protesters took part in Wednesday's marches, which erupted into clashes with security forces and armed groups of government supporters.

A 17-year-old boy and a 23-year-old woman died after being shot in the head, and Maduro's camp said a soldier outside Caracas was also killed.

The opposition accuses Maduro of letting state forces and gangs of armed thugs violently repress demonstrators.

Interior Minister Nestor Reverol said one person had been arrested for the soldier's death, which he called a pre-meditated "act of fascism."

Pressure on Maduro has been mounting since 2014, as falling prices for Venezuela's crucial oil exports have sent the economy into a tailspin.

Recent moves by Maduro to tighten his grip on power and ban Capriles from politics have escalated the country's political and economic crisis and sparked international outcry.

UN Secretary-General Antonio Guterres on Thursday urged all sides in Venezuela to take steps to ease tensions.

"We call for concrete gestures from all parties to reduce polarization and create the necessary conditions to address the country's challenges," said Guterres in a statement.

The European Union added to the condemnation, calling the deaths of protesters "highly regrettable" and urging all sides to "de-escalate."

The deepening of Venezuela's economic and political crisis since late March has galvanized the often divided opposition in their efforts to force Maduro from power.

"I don't have any food in the fridge," protester Jean Tovar, 32, told AFP on Wednesday as he held rocks in his hands ready to throw at military police in Caracas.

"I have a two-year-old son to support and I am unemployed, and it is all Maduro's fault."

The president in turn has urged his supporters, the military, and civilian militias to defend the socialist "Bolivarian revolution" launched by his predecessor Hugo Chavez in 1999.

The opposition has called for the military -- a pillar of Maduro's power -- to abandon him.

But the defense minister, General Vladimir Padrino Lopez, has pledged the army's "unconditional loyalty" to Maduro.

The president accuses the opposition of inciting a "coup" backed by the United States.

According to a survey by pollster Venebarometro, seven in 10 Venezuelans disapprove of Maduro, whose term does not end until 2019.

Maduro said Wednesday he was ready to face his opponents at the ballot box.

"I want to have elections soon... to seek a peaceful path so the revolution can put the conspirators, murderers and interventionist right-wingers in their place," he told a rally of supporters in central Caracas.

Regional elections due in December were indefinitely postponed and there is still no date for local polls due this year. The next presidential election is due in December 2018.