Venezuelan President Hugo Chavez's imminent departure for more cancer surgery in Cuba has thrown his re-election campaign into uncertainty and once again shaken the socialist leader's passionate supporters.

Though the 57-year-old former soldier looked stoic and played down the dangers of his latest condition, the announcement inevitably raises questions over his ability to stand for the October 7 presidential vote - or rule beyond it.

Chavez, who has dominated Venezuela since taking office in 1999 and whose fierce anti-U.S. rhetoric has turned him into one of the world's best-known leaders, said he will head back to Havana for the next operation by the weekend.

He said a two-centimetre lesion had been discovered in his pelvis where surgeons removed a baseball-sized tumour in June, and that there was a high probability it was malignant.

His challenger for the presidency in October, opposition candidate Henrique Capriles, was quick to wish Chavez well.

I wish my rival a successful operation, a quick recovery and a long life, Capriles, the 39-year-old governor of Miranda state, said on Wednesday in a careful tweet.

Capriles, who won the Democratic Unity coalition's primary this month, does not want to give any impression he is exulting in Chavez's health problems. He wants to focus on issues such as crime and unemployment, rather than a head-to-head battle.

Chavez's new surgery could hardly have come at a worse time for the government because Capriles - a centre-left politician who exudes youth and energy - picked up momentum from his primary win and the campaigns were just starting to heat up.

BONDS RISE

With investors always looking for signs of a more market-friendly government, the prices of Venezuela's widely traded bonds rose after word that Chavez needed more surgery.

The Global 2027 bond saw its biggest daily price increase in five months on Wednesday, climbing by 2.125 percent to bid at 82.625 with a yield of 9.25 percent, while another popular note, the Global 2022, rose 2.75 percent to bid at 102.063.

Democratic Unity issued a statement calling for transparency from the government, saying rumours had spread so quickly because the public had little access to reliable information.

Telling the truth is a democratic duty to the Venezuelan people - an even more important duty when it concerns the health of the person who is leading the government and has offered his candidacy to continue in that role for six more years, it said.

Some of the president's closest allies, including those who had denied rumours of any downturn in Chavez's health over the weekend, did a quick U-turn, calling for calm in Venezuela.

Our message is for unity and faith that President Chavez will triumph over this latest obstacle life has put in his way, Information Minister Andres Izarra said. His connection with the people will help him overcome any health problem.

Underlining the secrecy surrounding Chavez's condition, Izarra was among those who had virulently condemned rumours he had gone to Cuba for treatment during Venezuela's long carnival weekend, calling it a dirty war by scum.

Chavez then unexpectedly appeared in a televised tour of a tractor factory to announce that he had, in fact, been to Havana for tests and would need to return there by the weekend.

'I'M NOT IMMORTAL'

He said the surgery would be supposedly less complicated than his two operations last year, and that he would be fine for the presidential race. Less reassuringly for supporters, he also talked gravely about the revolution continuing without him.

I'm a human being, he said. I'm not immortal.

Venezuela was awash with speculation and gossip.

At one extreme, some people suggest Chavez invented the cancer to attract sympathy and create the image of a conquering return to health. Others expect him to die this year.

Cancer experts said the specific word chosen by Chavez - lesion - was deliberately opaque and could cover a wide variety of medical scenarios. The president has repeatedly said there was no metastasis, and that no one should be alarmed.

One medical source close to the team treating Chavez in Venezuela said he had been suffering a condition called tumour lysis, or cell breakdown, and agreed there was no metastasis.

Critics say Chavez should name a temporary replacement, but that is unlikely given that he chose to govern from his hospital bed in Havana during his extended absences last year.

Having dominated politics for so long, Chavez has no clear successor. Vice President Elias Jaua, who would theoretically take over if Chavez is incapacitated, does not have much clout in the ruling Socialist Party or with the military.

The Eurasia Group risk consultancy said the government had no real Plan B, meaning Chavez was still the candidate for the election and would remain so unless he was physically unable.

Chavez will be reluctant to cede power, and if Chavez were to step aside temporarily and anoint a successor to run in his place, it would send a very negative signal about his health prognosis and, more importantly, his power, it said.

Before Tuesday's announcement, polls gave Chavez an edge over Capriles at the vote, partly thanks to a massive program of state spending on social projects that appeal to his supporters among the poor majority. But about a third of Venezuelans remain undecided, so everything is still to play for.

(Additional reporting by Eyanir Chinea, Mario Naranjo, Marianna Parraga and Girish Gupta; editing by Mohammad Zargham)