Venezuela's President Hugo Chavez has died aged 58, after 14 years in power.
Chavez had been seriously ill with cancer for more than a year, undergoing several operations in Cuba, and had not been seen in public for several months.
Foreign Minister Elias Jose Jaua Milano declared seven days of mourning and said Chavez's body would lie in state until his funeral on Friday.
Vice-President Nicolas Maduro would assume the presidency until an election was called within 30 days, he added.
"It is the mandate that Comandante President Hugo Chavez gave us," Jaua told state television, adding that Maduro would also be the candidate of the governing United Socialist Party (PSUV).
It was not immediately clear when the election would take place.
Chavez's illness prevented him from taking the oath of office after he was re-elected for a third term in October and the President of the National Assembly, Diosdado Cabello, had been expected by some to take over as interim president in the event of his death.
However, he was not among the political and military leaders who flanked the vice-president when he announced Chavez's death.
In Tuesday's televised address, a tearful Maduro said the president had passed away "after battling a tough illness for nearly two years".
"We have received the toughest and tragic information that... Comandante President Hugo Chavez died today at 16:25 (20:55 GMT)," he added.
Earlier, he said the Venezuelan leader had a severe respiratory infection and had entered "his most difficult hours".
The death of Hugo Chavez, the vanguard of what he called "21st Century Socialism", sends ripples not just through the Venezuelan people, but across Latin America and beyond.
In particular, the impact of his loss will be felt most keenly in Cuba, Bolivia and Ecuador, his closest allies in the region.
The updates on his health in recent days had seemingly been aimed at preparing the Venezuelan people for the worst, with each bulletin more serious than the last.
Now Chavez will take on iconic status as his revolution looks for a route forward without him, the man it was designed by and constructed around.
But his millions of followers in Venezuela will take some comfort from the fact that it wasn't the failed coup in 2002, nor the repeated efforts at the ballot box, but rather ill health - or for many of his devotees, the hand of God - that took Chavez away from them.
He spoke of a plot against Venezuela, saying he had no doubt that Chavez's cancer, first diagnosed in 2011, had been induced by foul play by Venezuela's enemies - the US promptly rejected the accusations as "absurd".
He said a scientific commission could one day investigate whether Chavez's illness was brought about by what he called an enemy attack.
Struggling to hold back tears, Maduro called on the nation to close ranks after their leader's demise.
He said the government had deployed the armed forces and police nationwide "to accompany and protect our people and guarantee the peace".
Earlier, he said he had expelled two US diplomats from the country for spying on Venezuela's military.
A statement by the military said it would protect the sovereignty, integrity and security of the country. It would remain loyal to the vice-president and to parliament, it added, urging people to remain calm.
Jaua said a procession would carry Chavez's body to the Military Academy in Caracas on Wednesday, where it will lie in state until Friday to allow his supporters to pay their respects.
Jaua added that the official funeral attended by foreign heads of state would take place at 10:00 local time (14:30 GMT) on Friday, and called on Chavez's supporters to wear clothes in the three colours of the Venezuelan flag in his honour.
The US described the death as a "challenging time", reaffirming what it described as its support for the Venezuelan people and its interest in developing a constructive relationship with Caracas.
Timeline: Hugo Chavez
1954: Born 28 July in Sabaneta, Barinas state, the son of schoolteachers
1975: Graduated from Venezuelan Academy of Military Sciences
1977: Becomes involved in revolutionary movements within the armed forces
1981: Returns to the military academy as a teacher
1992: Leads doomed attempt to overthrow government of President Carlos Andres Perez, jailed for two years
1994: Relaunches his party as the Movement of the Fifth Republic
1999: Takes office after winning 1998 election
2002: Abortive coup. Returns to power after two days
2011: Reveals he is being treated for cancer
2012 (October): Re-elected for another six-year term
2012 (December): Has fourth cancer operation in Cuba
2013 (February): Returns to Venezuela to continue treatment
Obituary: Hugo Chavez
Praise for 'tough' BBC interviewer
"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," said a statement from the White House.
Russia's permanent representative to the UN, Vitaly Churkin, said Chavez had been a great politician, for his country, for Latin America and the world.
The government of Cuba declared three days of national mourning. In a statement read out on state television, it said Mr Chavez had "stood by Fidel [Castro] like a true son", referring to Cuba's former president, who stepped down in 2006 due to ill-health.
In Argentina, President Cristina Fernandez de Kirchner suspended all activities after the death was announced.
Both she and her late husband, Nestor Kirchner, were close friends of the firebrand Venezuelan leader.
In Peru, Congress held a minute of silence in his honour. Bolivia's President Evo Morales said he was leaving immediately for Caracas.
The Ecuadorian government said it felt the loss as its own, and hoped its neighbours could carry on Chavez's revolution.
Analysts say Chavez's death could alter the political balance in Latin America - dealing a blow to leftist states while favouring more centrist countries.
There could also be an economic impact given that Venezuela sells oil at below market prices to some neighbouring countries, especially in the Caribbean.
UK Foreign Minister William Hague said he was "saddened" to learn of the death, saying Chavez had left a "lasting impression" on Venezuela.
The UK Foreign Office issued a travel advisory warning visitors to Venezuela: "This is a sensitive moment for the country. You are strongly advised to avoid any public gatherings."
One of the most visible, vocal and controversial leaders in Latin America, Hugo Chavez won the presidency in 1998 and had most recently won another six-year presidential term in October 2012.
His government has implemented a number of "missions" or social programmes, including education and health services for all. But poverty and unemployment are still widespread, despite the country's oil wealth.
Chavez was renowned for his flamboyant public speaking style, which he put to use in his weekly live TV programme, Alo Presidente (Hello President), in which he talked about his political ideas, interviews guests and sings and dances.
Last May, the former army paratrooper said he had recovered from an unspecified cancer, after undergoing surgery and chemotherapy in 2011 and a further operation in February 2012.
However, in December, he announced he needed further cancer surgery in Cuba, and named Maduro as his preferred successor should the need arise.
Chavez remained out of public view, finally returning to Venezuela in February.