Venezuelan President Hugo Chavez has died at the age of 58 after losing his long battle with cancer.
The country's vice president Nicolas Maduro fought back tears as he announced the death in a national television broadcast.
He said Mr Chavez died at 4.25pm local time "after battling a tough illness for nearly two years".
Mr Maduro also said the army and police had been deployed "to accompany and protect our people and guarantee the peace".
The country's foreign minister Elias Jaua declared seven days of national mourning and said a wake for Mr Chavez will be held until Friday, when his official funeral will take place.
He said Mr Maduro would take over the presidency until fresh elections can take place in around 30 days.
President Barack Obama reaffirmed American "support for the Venezuelan people and its interest in developing a constructive relationship with the Venezuelan government".
"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," he added.
Foreign Secretary William Hague said: "I was saddened to learn of the death of President Hugo Chavez today. As President of Venezuela for 14 years he has left a lasting impression on the country and more widely."
Mr Chavez, whose condition had been said to be deteriorating in recent days, had been undergoing cancer treatment in Cuba on and off since June 2011 - when he was first diagnosed with the illness.
The announcement came just hours after Mr Maduro announced the government had expelled two US diplomats from the country.
He said "we have no doubt" that Mr Chavez's illness was induced by foul play by "the historical enemies of our homeland".
Hundreds of Venezuelans poured into the streets of Caracas, with many crying, hugging each other, or shouting slogans of support for Mr Chavez.
Clusters of women with tears streaming down their faces clung to each other and wept near the Miraflores presidential palace. Some wore T-shirts with slogans that read "Go forward commander!"
Nearby, grime-faced men pumped their arms in the air while shouting "Long live Chavez! Long live Chavismo!"
People also gathered outside the military hospital where Chavez died. Soldiers in riot gear stood shoulder to shoulder guarding the complex.
"I feel such big pain I can't even speak," said Yamilina Barrios, a 39-year-old office worker weeping at a street corner. "He was the best thing the country had ... I adore him. Let's hope the country calms down and we can continue the tasks he left us."
The government announced late on Monday that Mr Chavez's condition was "very delicate" due to a "new, severe" respiratory infection.
Mr Chavez had not been seen in public or heard since undergoing a fourth round of surgery in Cuba on December 11 in the pelvic area.
The government said he returned home on February 18, and had been confined to Caracas' military hospital ever since.
During more than 14 years in office, Mr Chavez routinely challenged the status quo at home and internationally.
The fiery populist leader declared a socialist revolution in Venezuela, crusaded against US influence, championed a leftist revival across Latin America, and over time, gradually placed all state institutions under his personal control.
Following his death a period of mourning was also declared in Cuba, where Mr Chavez was hailed as a "true son" to Fidel Castro.
But the former army paratrooper commander, who rose to fame by launching a failed 1992 coup, never groomed a successor.
His death sets up a snap presidential election after his illness prevented him from taking the oath of office when he was re-elected last year.
Under the constitution, the head of Congress, Diosdado Cabello, would assume the interim presidency.
However, Mr Maduro is Mr Chavez's self-anointed successor and has been holding the reins since the president's health took a turn for the worse.
The man Mr Chavez defeated in October's presidential elections, Miranda state Governor Henrique Capriles, is expected to represent the opposition in any new national polls.
He called for unity and offered his condolences to Mr Chavez's family and supporters.
Venezuela's defence minister pledged the military would remain loyal to the constitution in the wake of Mr Chavez's death.
Admiral Diego Molero appeared on national television and appealed for "unity, tranquillity and understanding" among Venezuelans.
Sky's Dominic Waghorn said Mr Chavez "used a mixture of brute force, persuasion, passion and charisma to keep himself in power".
"Such was the adoration and devotion that mainly the poor in Venezuela felt for him that he was seen as this almost sort of religious figure, and his loss now leaves a huge void in Venezuelan politics.
"A lot of people say he is irreplaceable."
