* Treatment due to begin on Sunday
* President seeking re-election in October
* Says he will return in several days (Updates with Chavez's departure for Havana, edits)
By Daniel Wallis
CARACAS, March 24 (Reuters) - Venezuelan President Hugo Chavez flew back to Cuba on Saturday to begin radiation treatment for cancer, but said he was in good shape and would be back home in several days.
The socialist leader's latest trip to Havana will heighten anxiety among supporters worried about his health, fan rumors of a power struggle among his top aides, and leave Chavez absent just as his election rival is stepping up a campaign tour.
Since making a triumphant return to Caracas from Cuba a week ago after a third operation in less than a year to remove a malignant tumor from his pelvis, the 57-year-old had been saying he would start radiation therapy soon.
But until Saturday he had not revealed whether he would go to Cuba or stay in Venezuela for the treatment, which is expected to leave him weaker during his campaign to win a new six-year term at an Oct. 7. vote. Chavez has no clear successor.
"I have decided, on the recommendation of my medical team and my political team, to begin the radiation treatment tomorrow," Chavez said during a televised cabinet meeting on Saturday before leaving for Maiquetia international airport.
As he addressed white-clad troops gathered on the runway there, government ministers looked on, grim faced. He said he might have more radiation treatment in Venezuela in the future.
Little is known about what kind of cancer the president has, nor how serious it is. So big questions remain about his future.
Political risks in Venezuela:
Chavez has dominated Venezuelan politics for the last 13 years, and his illness has shocked voters in South America's biggest oil exporter in the run-up to the election.
Some have questioned how fit he would be to govern if he won, and his treatment is expected to stop him from conducting the kind of man-on-the-street campaign that has worked so well in the past to help him drive forward his leftist "revolution."
Sunday will mark four weeks since Chavez's most recent surgery at Havana's high security Cimeq Hospital.
"Thank God, yesterday they removed the last stitches that were left from the operation. All very good," he said.
"I'm walking much better ... without any complications. A month after the operation, we're ready for the radiation treatment, which will last for around four or five weeks."
The opposition has called on Chavez to name a formal replacement during his absences in Havana - a proposal he rejects, preferring to govern from his hospital bed.
"I'm sure this decision (to return to Cuba) will be criticized by some poisonous opposition spokesmen," he said.
"But I am certain in my heart that the great majority of Venezuelans are with me and are committed to my full recovery. ... I'll be back in a few days."
Chavez's weaker figure contrasts sharply with the youthful, energetic image presented by his rival, basketball-loving 39-year-old Miranda state governor Henrique Capriles.
But most recent polls have given Chavez a healthy lead over Capriles, mainly thanks to massive state spending on popular social projects, as well as his charisma and strong emotional connection with the country's poor majority.
Venezuelan opinion polls, however, have long been highly divergent and controversial, with accusations of bias filling the airwaves every time a new one is published.
Three surveys that came out this month gave Chavez a big lead in voter intentions of between 52 and 57 percent, versus 22 to 34 percent for Capriles.
Then a fourth poll put the pair just one point apart: 46 percent for the president and 45 percent for his opponent.
"Haha. There are polls for all tastes," a pro-Chavez political scientist, Nicmer Evans, joked in a Twitter exchange this week with an opposition blogger who suggested some pollsters were mere "briefcase companies," while another might exist only in its owner's imagination.
Before ending Saturday's cabinet meeting and preparing to leave for Cuba, the president taunted the opposition again, calling them the candidates of the far-right who were determined to bring chaos to Venezuela, but would lose at the ballot box.
"You gentlemen of the opposition are going to stay in the opposition for another 500 years, at least," he said.
Being in Havana may give Chavez the chance to meet Pope Benedict, who is visiting the communist-led Caribbean island on Monday after a three-day stop in Mexico.
One Venezuelan journalist, who has been breaking news on Chavez's cancer saga in the absence of detailed official information, said the Pope had agreed to a request for a private meeting with Chavez.
Capriles has sought to avoid entering into speculation over the president's health, but has been critical of Chavez for not being treated at home. "What message does that send to the ordinary Venezuelan?" he asked Reuters in a recent interview.
Chavez prefers going to Cuba because he is guaranteed greater privacy on the tightly controlled island. He also leans heavily on the counsel and support of his political mentor and friend Fidel Castro. (Additional reporting by Deisy Buitrago; editing by Andrew Cawthorne, Todd Eastham and Eric Beech)