CARACAS (AFP) - Venezuelan President Hugo Chavez lost his battle with cancer on Tuesday, his death silencing the leading voice of the Latin American left and plunging his oil-rich nation into an uncertain future.

Vice President Nicolas Maduro, who struggled to stifle tears as he announced Chavez's passing, said the government had deployed the armed forces and police "to accompany and protect our people and guarantee the peace."

Mr Chavez had named Mr Maduro as his heir, but the Venezuelan opposition is sure to press for fresh elections and tensions have been mounting over government allegations that its domestic rivals are in league with its foreign foes.

Shortly before Mr Chavez's death was announced, senior officials had accused Venezuela's enemies of somehow giving the 58-year-old leftist the cancer that eventually killed him, and two US military attaches were expelled.

Mr Chavez was showered with tributes by Latin American leaders, not just his leftist allies but also world figures like Brazil's Dilma Rousseff, who hailed him as a "great Latin American" and "a friend of the Brazilian people."

US President Barack Obama - often the target of Mr Chavez's anti-American scorn - was more circumspect, pledging the United States to support the "Venezuelan people" and describing Mr Chavez's passing as a "challenging time."

"As Venezuela begins a new chapter in its history, the United States remains committed to policies that promote democratic principles, the rule of law, and respect for human rights," Mr Obama said in a short written statement.

He nevertheless expressed hope that US-Venezuelan relations would improve.

Under the constitution, elections must be held within 30 days and National Assembly speaker Diosdado Cabello must take over as caretaker, but Mr Chavez had urged Venezuelans to back Mr Maduro if he was unable to continue.

Die-hard Chavista partisans gathered in Caracas' Plaza Bolivar - named after the independence hero whose legacy Chavez co-opted for his Bolivarian Revolution - weeping, waving portraits and chanting his name.

One of Mr Chavez's daughters, 32-year-old Maria Gabriela, wrote on Twitter: "I'm lost for words. Eternally, THANK YOU! Strength! We must follow his example. We must continue building the FATHERLAND! Farewell my daddy!"

Soldiers brought the Venezuelan flag down to half-staff at the Caracas military hospital, where senior figures in Mr Chavez's 14-year-old administration gathered before the cameras of state television to break the news.

"We have received the toughest and tragic information that... comandante President Hugo Chavez died today at 4:25 pm," Mr Maduro said.

"Long live Chavez!" the officials shouted at the end of his announcement.

Defence Minister Diego Molero, surrounded by top military officers, said the armed forces would defend the constitution and respect Mr Chavez's wishes.

Mr Chavez had checked into the hospital on Feb 18 for a course of chemotherapy after spending two months in Cuba, where in December he had undergone his fourth round of cancer surgery since June 2011.

The once ubiquitous symbol of Latin America's "anti-imperialist" left disappeared from public view after he was flown to Cuba on Dec 10, an unprecedented absence from the public eye that fueled all manner of rumors.

The government sent mixed signals about the president's health for weeks, warning one day that he was battling for his life, yet insisting as recently as last weekend that he was still in charge and giving orders.

And the opposition repeatedly accused the government of lying about the president's condition.

A new election could offer another shot at the presidency to Mr Henrique Capriles, the opposition leader who lost to Chavez in October. He took to Twitter to call for unity.

"My solidarity is with the entire family and followers of President Hugo Chavez, we call for Venezuelan unity at this moment," Mr Capriles wrote.

Mr Chavez will be mourned by many of the country's poor, who revered the self-styled revolutionary for using the country's oil riches to fund popular housing, health, food and education programs.

And like-minded Latin American leaders like Cuba's Raul Castro, Ecuador's Rafael Correa and Bolivia's Evo Morales lost a close friend who used his diplomatic muscle and cheap oil to shore up their rule.

Mr Chavez died five months after winning an October election, overcoming public frustration over a rising murder rate, regular blackouts and soaring inflation.

The opposition had accused Mr Chavez of misusing public funds for his campaign and dominating the airwaves while forcing government workers to attend rallies through intimidation.

He missed his swearing-in for a new six-year term on Jan 10, but the Supreme Court approved an indefinite delay.

First elected in 1998, Mr Chavez had since worked to consolidate his power and make his revolution "irreversible."

But his policies drove a wedge into Venezuelan society, alienating the wealthy with expropriations while wooing the poor with social handouts.