The announcement by Juan Guaido, 35, head of Venezuela's opposition-led legislature, came amid a fresh wave of deadly street clashes on Wednesday.

He declared himself acting leader of the oil-rich nation, which has lurched into economic chaos and violence under Maduro, 56.

The Socialist government responded by warning that the top military leadership would come out Thursday "in support of the constitutional president", Defense Minister Vladimir Padrino Lopez said.

He added the military would show "backing for the sovereignty" of Venezuela.

That was a warning shot to Maduro's foreign critics as they rallied behind Guaido.

US President Donald Trump declared Maduro "illegitimate" and called the National Assembly legislature "the only legitimate branch of government."

A furious Maduro responded to the US move by breaking off diplomatic ties with the "imperialist" US government, giving its diplomats 72 hours to leave.

The US State Department said it did not recognize Maduro as president anymore so his order meant nothing.

France added its support for Guaido, also branding Maduro "illegitimate" and calling for "the restoration of democracy" in Venezuela.

Maduro's key ally Russia meanwhile denounced Guaido's bid as a "usurpation" of power and condemned what it called foreign "interference" in Venezuela.

"This is a direct path to lawlessness and bloodshed," the Russian foreign ministry said in a statement.

Maduro's key financial backer China also weighed. Foreign ministry spokeswoman Hua Chunying said China opposes "interference" in Venezuelan affairs and called for a "political resolution."

13 killed in clashes 

Guaido's pronouncement capped days of unrest that have seen 13 people killed, the Venezuelan Observatory of Social Conflict told AFP.

They were the first mass street protests in Venezuela since a peak of violence in April and July, 2017, when 125 people died in clashes.

Riot police fired tear gas and rubber bullets at opposition protesters in one Caracas suburb, while television pictures also showed armoured vehicles in the capital.

Thousands of Maduro supporters, many wearing red, converged outside the presidential palace, Miraflores, to oppose what they said was a US-backed opposition coup attempt.

Elsewhere in Caracas, tens of thousands of opposition supporters rallied, dressed in white and waving Venezuelan flags.

"Guaido, friend, the people are with you," they chanted.

Maduro hit back in a speech from a balcony at the presidential palace, lashing out at Trump.

He condemned what he called the US leader's "extremist policy" as "a very serious folly."

"Trying to impose a government by extra-constitutional means, we cannot accept that."

UN Secretary General Antonio Guterres appealed for dialogue to avoid the political crisis spiralling out of control.

"What we hope is that dialogue can be possible, and that we avoid an escalation that would lead to the kind of conflict that would be a disaster for the people of Venezuela and for the region," he said at the World Economic Forum in Davos.

- Military support key -

Maduro was reelected in May in snap elections boycotted by the opposition and denounced around the world as illegitimate.

He has held onto power in the face of mass protests, international pressure and opposition efforts to oust him.

Analysts at the Eurasia Group consultancy said his opponents could only prevail if the top military command abandon him.

They said Trump might consider further economic sanctions against Venezuela, from which the United States buys more than a million barrels of oil a day.

Guaido called on the military to join "the side of the constitution" and offered an amnesty to any Maduro allies who would defect.

He was elected president of the National Assembly -- the only major state pillar not controlled by the government -- in December.

Since then, he has managed to rally a divided opposition.

"I have faith and hope in Guaido, a young lad who can help us to go forward," one demonstrator in Caracas, 49-year-old Florangel Rodriguez told AFP.

- Venezuela economic crisis -

Maduro has presided over a deepening economic crisis that has left millions in poverty as the country faces shortages of basic necessities such as food and medicine.

Some 2.3 million people have fled the country since 2015, according to the United Nations. The International Monetary Fund says inflation will hit 10 million percent this year.

Guaido called for the protests two weeks ago in a bid to rally support behind his aim to remove Maduro, set up a transitional government and hold elections.

A dozen regional players have also backed Guaido, including Brazil, Argentina, Colombia and Canada.

Mexico and Cuba stood firm in support of Maduro.

Spanish Foreign Minister Josep Borrell said fresh elections in Venezuela were the "only way out" of the crisis.