By Andrew Cawthorne
CARACAS (Reuters) - Venezuela will set up a formal inquiry into claims that deceased President Hugo Chavez's cancer was the result of poisoning by his enemies abroad, the government said.
Foes of the government view the accusation as a typical Chavez-style conspiracy theory intended to feed fears of "imperialist" threats to Venezuela's socialist system and distract people from daily problems.
Acting President Nicolas Maduro vowed to open an investigation into the claims, first raised by Chavez after he was diagnosed with the disease in 2011.
"We will seek the truth," Maduro told regional TV network Telesur. "We have the intuition that our commander Chavez was poisoned by dark forces that wanted him out of the way."
Foreign scientists will be invited to join a state committee to probe the accusation, he said.
Maduro, 50, is Chavez's handpicked successor and is running as the government's candidate in a snap presidential election on April 14 that was triggered by the president's death last week.
He is trying to keep voters' attention firmly focused on Chavez to benefit from the outpouring of grief among his millions of supporters. The opposition is centering its campaign on portraying Maduro, a former bus driver, as an incompetent who, they say, is exploiting Chavez's demise.
"Let's take the president (Chavez) away from the political debate, out of respect for his memory, his family, his supporters," opposition candidate Henrique Capriles' campaign chief Henri Falcon told reporters.
Polls from before Chavez's death gave Maduro a lead over Capriles of more than 10 percentage points. Capriles lost to Chavez by 11 percentage points in October.
Capriles has tried to jump-start his campaign with accusations that Maduro and other senior officials lied about the details of Chavez's illness, hiding the gravity of his condition from Venezuelans.
That sparked a torrent of attacks, with senior government officials using words like "Nazi" and "fascist" to describe Capriles, who has Jewish ancestors.
In a televised message, Information Minister Ernesto Villegas read a letter to the "sick opposition" from the late president's daughter, Maria Gabriela Chavez, who has at times been viewed as a possible future successor.
"Stop playing with the pain of a nation and a devastated family," she wrote. "It is unfair, inhuman, unacceptable that they now say we were lying about the date of his (death) ... Focus on politics, don't play dirty."
Capriles was quick to respond with a flurry of tweets.
"Never, in all these years, have I offended the president or his family. If one word has been taken thus by his family, I'm sorry," he wrote on Twitter.
"I don't offend families as they have mine. They have even called me a Nazi, when my great-grandparents were murdered in a Nazi concentration camp," he added, referring to the government.
In an increasingly acrimonious campaign, both sides on Tuesday accused each other of planning violence.
The opposition displayed photos circulating on the Internet showing an assault rifle and a pistol being held up to a TV screen that was broadcasting Capriles' face.
They also said there were indications of plans to attack Capriles when he was scheduled to register his candidacy on Monday. In the end, aides went instead.
Government spokesmen repeated accusations that opposition activists planned to disrupt Maduro's campaign.
Trying to discredit Capriles, they waved photos of a plush New York apartment they said belonged to him, and displayed copies of university documents that they said showed he never completed a law degree.
Capriles, a 40-year-old, business-friendly regional governor running for the opposition's Democratic Unity coalition, is trying to disassociate Maduro from Chavez in voters' minds.
"He's attacking Nicolas Maduro, saying Nicolas is not Chavez," senior Socialist Party official and Maduro's campaign chief Jorge Rodriguez said.
"Of course Nicolas isn't Chavez. But he is his faithful, responsible, revolutionary son. All these insults and vilification are going to be turned into votes for us," he said.
Tuesday was the last day of official mourning for Chavez, although ceremonies appear set to continue. His embalmed body was to be taken in procession to a military museum on Friday.
Millions have filed past Chavez's coffin to pay homage to a man who was adored by many of the poor for his humble roots and welfare policies, but was also hated by many people for his authoritarian style and bullying of opponents.
Though Maduro has spoken about combating crime and extending development programs in the slums, he has mostly used his frequent appearances on state TV to talk about Chavez.
The 58-year-old president was diagnosed with cancer in his pelvic region in June 2011 and underwent four surgeries before dying of what sources said was metastasis in the lungs.
Maduro said it was too early to specifically point a finger over Chavez's cancer, but noted that the United States had laboratories with experience in producing diseases.
"He had a cancer that broke all norms," Maduro told Telesur. "Everything seems to indicate that they (enemies) affected his health using the most advanced techniques."
Maduro has compared his suspicions over Chavez's death with allegations 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 Venezuela's independence hero Simon Bolivar died of poisoning by his enemies in Colombia in 1830.
OPPOSITION'S UPHILL FIGHT
The National Assembly was to debate this week a proposal by pro-government legislators to hold a referendum - possibly also on April 14 - on whether he should be buried at the ornate National Pantheon building in Caracas.
Opponents are outraged at the prospect of a referendum stoking the emotion around Chavez at the same time as the presidential vote.
Besides the wave of sympathy for Chavez, the opposition faces a well-financed state apparatus, institutions packed with government supporters, and problems within its own rank-and-file, still demoralized over October's presidential election defeat and a mauling at gubernatorial polls in December.
At stake in the election is the future of Chavez's leftist "revolution," the continuation of Venezuelan oil subsidies and other aid crucial to the economies of left-wing allies around Latin America, from Cuba to Bolivia.
The OPEC nation boasts the world's largest oil reserves.
Though there are hopes for a post-Chavez rapprochement between Venezuela and the United States, a diplomatic spat worsened on Monday when Washington expelled two Venezuelan diplomats in a tit-for-tat retaliation.
(Additional reporting by Marianna Parraga, Simon Gardner, Pablo Garibian and Enrique Andres Pretel; Editing by Daniel Wallis and Stacey Joyce)