* Shots fired at weekend opposition campaign stop
* Capriles' camp blames pro-Chavez militants
* Venezuelan leader faces cancer radiation treatment (Adds bonds, opinion poll, details)
By Andrew Cawthorne
CARACAS, March 5 (Reuters) - Venezuela's government and opposition blamed each other on Monday for a violent melee at a campaign stop by presidential challenger Henrique Capriles in which several people were injured by bullets.
The flare-up in a poor neighborhood of Caracas underlined the potential for trouble during what is shaping into a close-fought race between Capriles and President Hugo Chavez, who is seeking to extend his 13-year rule in the Oct. 7 election.
Capriles' camp said red-shirted members of Chavez's ruling Socialist Party opened fire when the opposition leader and his supporters were walking through the Cotiza neighborhood.
Two supporters, including the son of an opposition legislator standing near Capriles, were injured, they said.
With Chavez in Cuba for cancer treatment, senior government officials called the opposition's account a lie, saying Capriles' security guards began Sunday's shooting and injured four people.
Both sides repeatedly played footage with different angles and takes of the incident on their TV stations to back up their version of the events, with the crack of bullets sounding as people were seen fleeing.
"While this government debates with weapons, we debate with ideas," Capriles, the 39-year-old center-left governor of Miranda state, said after the incident. "What are they scared of?"
Interior Minister Tareck El Aissami promised an investigation but said Miranda state police, operating without permission outside their state, had attacked government supporters involved in another activity.
"They were the promoters of the violence," he said. "They decided to mount this show during the dull activity of the candidate of the right, who could not even draw 10 people."
Added to the deep political polarization under Chavez, Venezuela has one of the worst crime rates in the world. Weapons abound - especially in poorer areas like Cotiza - and the potential for violence is high.
Another opposition leader, Maria Corina Machado, had shots fired at her entourage in November while campaigning in a Caracas shantytown ahead of the opposition Democratic Unity primary that Capriles won in February.
CHAVEZ FACES RADIATION TREATMENT
The fallout from Sunday's shooting came as Venezuelans reacted to news that Chavez faces more radiation treatment even as he seeks reelection.
After being stricken by cancer last year, Chavez has had two tumors removed - the latest last week - but insists there is no metastasis or spread of the disease.
Chavez said he needs the radiation treatment, which could leave him queasy and weak as he faces a campaign. Having wrongly claimed to be completely cured last year, some Venezuelans doubt his latest self-prognosis and believe he could be dying.
Anxious supporters are passionately backing him to recover and win the election, chanting "Uh! Ah! Chavez will be cured!" in a variation of their usual "Uh! Ah! Chavez is not going to leave!" at rallies held in his honor.
While the image of a sick candidate may win some sympathy, most political analysts said it would more likely undermine Chavez's campaign and play into the hands of an opposition candidate exuding youth and energy.
Driven by rising market expectations of a more business-friendly government, Venezuelan bonds continued to rise on news of Chavez's health woes. The benchmark dollar-denominated 2027 global bond rose another 1.57 percent on Monday.
Chavez still remains hugely popular, however, especially among the poor who have benefited from massive spending on welfare policies fuelled by the OPEC member's oil income.
In the latest survey available, taken after Chavez's return to Cuba at the end of February for new surgery, pollster Hinterlaces said 52 percent of Venezuelans intended to vote for Chavez in October while 34 percent backed Capriles.
Though Venezuela's polls are always hotly-debated and accompanied by accusations of bias, the survey underlined what a mammoth task Capriles faces.
"For Venezuelans, the national head of state must continue his political career even though he is sick," Hinterlaces director Oscar Schemel said, according to local media.
During Sunday's fracas in Cotiza, private, pro-opposition TV channel Globovision - long hated by Chavez supporters - also said its crew was attacked and equipment was stolen.
While Venezuelan politics are constantly dogged by low-level violence, the South American nation has also seen major unrest at various points in the last few decades.
Hundreds were killed when Chavez led a failed 1992 coup attempt. Ten years later, he was briefly ousted from the presidency after opposition protests ended in bloodshed, killing more than a dozen people, outside his Miraflores palace in Caracas. (Editing by Girish Gupta and Paul Simao)