Caracas (AFP) - Venezuela erupted Monday in a fifth day of violent protests against President Nicolas Maduro as the United States voiced "grave concern" over moves to stifle one of his main opponents.

In the latest clashes in the once-booming oil exporter, riot police in Caracas fired tear gas at stone-throwing demonstrators -- whose leaders vowed not to let up the pressure on Maduro.

"This is a battle of resistance. We will see who gets tired first: us of fighting, or them of repressing," said the deputy speaker of the opposition-majority congress, Freddy Guevara.

The streets of Caracas and several other Venezuelan cities have seen running battles in recent days as protesters have rallied demanding elections.

Police fired water cannons, tear gas and rubber bullets at demonstrators, who hurled rocks and Molotov cocktails.

On Monday, several patients including a newborn baby had to be evacuated from a medical clinic in the Las Mercedes district after it was struck by tear gas canisters, a doctor there, Luis Montanes, told reporters.

One protester was killed on Thursday. Dozens of people have been wounded or arrested.

- US: 'grave concern' -

Maduro is fighting efforts to force him from power over an economic crisis marked by severe shortages and the world's highest inflation.

Pressure rose on him last week after two Supreme Court rulings that stripped the legislature's power. The court later reversed the rulings amid an outcry.

But the crisis deepened Friday when authorities banned senior opposition figure Henrique Capriles from holding public office for 15 years, blocking him from running against Maduro in next year's election.

The United States on Monday expressed "grave concern" about Capriles's ban.

"We urge President Maduro to reconsider the decision to bar Capriles and ensure Venezuelans can exercise their right to elect their representatives in free and fair elections," the US State Department said in a statement.

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The European Union earlier added to the international pressure, raising alarm over the "ongoing escalation of tensions and violent confrontations."

- Maduro defiant -

Speaking at a meeting with allied leftist leaders in Cuba, Maduro repeated his claim that his old foe the United States was to blame for the crisis.

"The order has been given from Washington that there must be no dialogue in Venezuela, to make our country explode and give way for a foreign intervention," he said in a public address.

Maduro was in Havana for a meeting of foreign ministers of the leftist ALBA bloc, a Latin American group co-founded by his late mentor, Hugo Chavez.

Bolivia, Cuba, Ecuador, Nicaragua and several Caribbean countries issued a statement in his support.

They condemned "aggression and manipulation" against Venezuela that they said "threaten its sovereignty, independence and stability."

- 'Mother of all marches' -

The opposition is planning what Guevara called the "mother of all protest marches" on April 19 against Maduro.

Maduro's supporters announced they too would stage a major march that day in central Caracas.

The opposition demands, among other things, that authorities set a date for gubernatorial elections that have been postponed indefinitely.

Maduro said Sunday he was "eager" for the elections to go ahead so he could "hand a defeat to those people... so that they will stop the rioting and violence."

Guevara called the president a "liar."

"He knows he would lose" any election, the senior opposition lawmaker told AFP.

Demonstrator Carolina Moreno, a 39-year-old unemployed woman, said she was simply protesting for food.

She referred to the government's subsidized food program for poor neighborhoods.

"They give you one miserable bag of food a month," she told AFP.