The enemies of the government see the charge as a typical conspiracy theory Chavez style intended for the fears of "imperialist" threat to Venezuela's socialist system and distract people from everyday problems.
The interim president, Nicolas Maduro, agreed to open an investigation into the allegations, first raised by Chavez after being diagnosed with the disease in 2011.
"We will seek the truth," said Maduro regional television network Telesur. "We have the intuition that our commander Chávez was poisoned by the dark forces who wanted him out of the way."
Foreign scientists will be invited to participate in a state committee to investigate the charges, he said.
Maduro, 50, is the handpicked successor of Chavez and runs as government candidate in presidential elections early April 14 that was triggered by the death of the president last week.
He is trying to keep voters' attention firmly focused on Chavez to benefit from the outpouring of grief among his millions of followers. The opposition is focusing its campaign to portray Maduro, a former bus driver, as incompetent, they say, is the exploitation of the disappearance of Chavez.
"Let's take the president (Chavez) political debate, out of respect to his memory, his family, his supporters," said the campaign of opposition candidate Henrique Capriles' chief Henri Falcón journalists.
Polls prior to the death of Chavez gave an advantage over Capriles of Maduro more than 10 percentage points. Capriles Chavez lost by 11 percentage points in October.
Capriles has tried to launch their campaign of accusations that Maduro and other senior officials lied about the details of Chavez's illness, concealing the seriousness of his condition Venezuelans.
That sparked a torrent of attacks, with high-level government officials who use words like "Nazi" and "fascist" to describe Capriles, who has Jewish ancestry.
In a televised address, the information minister Ernesto Villegas read a letter addressed to the "opposition sick" of the late president's daughter, Maria Gabriela Chavez, who has sometimes been seen as a possible future successor.
"Stop playing with the pain of a nation and a family devastated," she wrote. "It is unfair, inhumane and unacceptable that now says they were lying about the date of his (death) ... Focus on policy, not foul play."
Capriles was quick to respond with a series of tweets.
"Never in all these years, I have offended the president or his family. If a word has been taken well by his family, I'm sorry," he wrote on Twitter.
"I do not offend the families as mine. Nazis have even called me when my grandparents were murdered in a Nazi concentration camp," he added, referring to the government.
ALLEGATIONS OF FLIGHT
In an increasingly bitter campaign, both parties on Tuesday accused each other of plotting violence.
The opposition shows photos circulating on the Internet showing an assault rifle and a pistol held to a television screen that was broadcasting the face Capriles.
He also said there was no indication of plans to attack when Capriles was programmed to register his candidacy on Monday. Finally, attendees were in place.
Government spokesmen repeated accusations that opposition activists were planning to disrupt campaign Maduro.
Trying to discredit Capriles, waving photos of a luxury apartment in New York, said he belonged, and displayed copies of documents said college was never completed a law degree.
Capriles, 40 years old, Business, regional governor candidate of the opposition coalition Democratic Unity, is trying to distance himself from Chavez Maduro in the minds of voters.
"He is attacking Nicolás Maduro Nicolás not saying Chavez", a senior official of the Socialist Party's campaign chief Jorge Rodriguez Maduro.
"Of course not Nicolas Chavez. But he is responsible for his faithful son, revolutionary. All these insults and slander be converted into votes for us," he said.
Tuesday was the last day of official mourning for Chavez, but ceremonies seem to continue. His embalmed body was taken in procession to a military museum on Friday.
Millions of people have marched Chavez coffin to honor a man who was adored by many of the poor of his humble roots and social welfare policies, but was hated by many people for his authoritarian style and intimidation of opponents.
Although Maduro spoke about the fight against crime and expanding development programs in poor neighborhoods, has been mainly used his frequent appearances on national television to talk about Chavez.
The president of 58 years old, was diagnosed with cancer in the pelvic region in June 2011 and underwent four operations before dying of what sources said was metastasis in the lungs.
Maduro said it was too early to point a finger specifically on cancer of Chavez, but noted that the United States had experienced laboratories that produce disease.
"I had a cancer that broke all the rules," Maduro said Telesur. "It seems that they (the enemies) affected their health using the most advanced techniques."
Maduro has compared his suspicions about the death of Chavez with accusations that Palestinian leader Yasser Arafat died in 2004 from poisoning by Israeli agents.
The case echoes Chavez's long campaign to convince the world that his idol and independence hero Simon Bolivar of Venezuela was poisoned by enemies in Colombia in 1830.
Opposition uphill struggle
The National Assembly was debating this week a proposal for lawmakers to hold a referendum - possibly April 14 - on whether he should be buried in the National Pantheon ornate building in Caracas.
Opponents are outraged at the prospect of a referendum stoke excitement surrounding Chávez, while the presidential vote.
In addition to the wave of sympathy for Chavez, the opposition faces a well-funded state apparatus, institutions full of government supporters, and the problems within their own rank and file, still demoralized by defeat presidential elections in October and rout in the polls for governor in December.
At stake in the elections is the future of the leftist Chavez "revolution", the continuation of Venezuelan oil subsidies and other aid crucial to the economies of the leftist allies throughout Latin America, from Cuba to Bolivia.
The OPEC nation has the world's largest reserves of oil.
Although there are hopes for a post-Chávez rapprochement between Venezuela and the United States, a diplomatic row escalated on Monday when Washington expelled two Venezuelan diplomats in tit for tat revenge.
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