A 17-year-old boy and a 23-year-old woman died after being shot on Wednesday during massive protests, and a soldier outside Caracas was said to have been killed, bringing to eight the number of people killed this month in a mounting political crisis.
Riot police fired tear gas to force back stone-throwing demonstrators as hundreds of thousands of people fed up with food shortages and demanding elections joined protest marches in Caracas and several other cities.
Thousands of Maduro's supporters held a counter-rally in central Caracas.
The opposition has accused Maduro of letting state forces and gangs of armed thugs violently repress demonstrators as he resists opposition pressure for him to quit.
Despite Wednesday's deadly violence, his opponents displayed their determination to ratchet up the pressure by calling for renewed protests on Thursday.
"Today there were millions of us," senior opposition leader Henrique Capriles told a news conference late on Wednesday.
"Tomorrow even more of us have to come out."
Pressure on the leftist president has been mounting since 2014, as falling prices for Venezuela's crucial oil exports have aggravated an economic crisis.
"I don't have any food in the fridge," said protester Jean Tovar, 32, who 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.
In Washington on Wednesday, Secretary of State Rex Tillerson said the United States is "concerned that the government of Maduro is violating its own constitution and not allowing the opposition to have their voices heard".