Hours later Mr Maduro addressed thousands of his supporters in Bolivar Avenue. The march has been broadcast live on national television.

"I call all the people to the streets in order to defend peace," he said.

He warned that his government would not give in to those he described as "fascists," including former president of neighbouring Colombia, Alvaro Uribe.

"Alvaro Uribe is behind this, financing and directing these fascist movements.

"He intended to use a Venezuelan television channel [NTN24] to do the same they did on 11 April 2002," Mr Maduro said, referring to an failed military coup against the late president, Hugo Chavez.

Mr Uribe, a centre-right politician, was a fierce enemy of Mr Chavez and accused the late president of supporting Colombia's largest rebel group, the Farc.

Maduro says the Venezuelan people must defend the "Bolivarian Revolution" launched by Hugo Chavez

Opposition demonstrators, including a movement known as Mothers in White, gathered at Las Mercedes neighbourhood in eastern Caracas.

Thousands of people, mostly wearing white, marched towards the Courts of Justice building and stopped for a memorial ceremony to the three demonstrators killed on Wednesday: Bassil da Costa, Juan Montoya y Robert Redman.

Student leader Enrique Altimari said the main aim of their "peaceful protest" was to "pay tribute to the victims".

He said the march would end before night fell, to avoid a repeat of the incidents of Wednesday.

The three victims were shot dead by unknown gunmen as the opposition marches came to an end.

"The streets at night are not a safe place for us. We would not be achieving any goal and would only fall in the trap set up by violent pro-government groups," Mr Altimari said.

They have denounced police brutality and blamed pro-government groups, known as "colectivos", for the deaths on Wednesday.

"Freedom for the prisoners," reads the banner held by opposition protesters.

But Venezuelan Prisons Minister Iris Varela has described the "colectivos" as "pillars in the defence of the nation".

The main opposition grievances are high inflation, crime and the shortage of some staples.

The government has blamed the shortages on "saboteurs" and "profit-hungry corrupt businessmen".

Venezuela has one of the highest murder rates in the world and is deeply politically polarised.

Mr Maduro was elected last April by a narrow margin, defeating the centre-right candidate, Henrique Capriles, who denounced electoral fraud.

A former union leader, Mr Maduro was a close ally of President Hugo Chavez, who died of cancer last March after 14 years in office.

Mr Capriles says the government's left-wing policies have led to economic collapse, including high inflation- 56.2% in 2013, according to official figures.