Haitians preparing to vote in the first presidential elections since January's devastating earthquake are coping with two threats -- the spectre of political violence and a spreading cholera epidemic.

Sunday's election will be particularly important for the Caribbean nation, as the next president will be tasked with overseeing the distribution of billions of dollars in foreign aid money.

The top candidates are at odds over how to use the funds, but all have criticized outgoing President Rene Preval for his handling of the quake's aftermath.

On Wednesday, an impromptu campaign rally in Port-au-Prince ended abruptly as supporters of a competing presidential contender arrived on motorcycle and began firing guns into the air.

The incident raised questions about whether the pending vote will be marred by violence, as has happened in past elections there.

While the presidential vote in 2006 was relatively calm, an election two years earlier was cancelled after the sitting president, Jean-Bertrand Aristide, was deposed in a coup.

Edmond Mulet, the United Nations representative in Haiti, described the "volatile political climate" as a Haitian tradition. Several candidates claim they have survived assassination attempts.

To making matters worse, the cholera epidemic in Haiti is spreading faster than many predicted and is likely to affect hundreds of thousands of people over the next year before it begins to slow, experts now predict.

Nigel Fisher, the UN humanitarian co-ordinator in Haiti, says the epidemic that began in mid-October had now spread to all 10 of the country's provinces.

He said while the official death toll sits at just over 1,300, it is likely the real toll is about 2,000, since little reliable data are coming in from remote areas.

Jon Kim Andrus, the deputy director of the Pan American Health Organization, said this week he expects there could be as many as 400,000 cholera cases over the next 12 months.

But about half of those cases -- 200,000 -- will occur over the next three months "because of the explosive nature of this cholera epidemic."

CTV's Lisa LaFlamme, reporting from Port-au-Prince, said it appears that the medical clinics set up in the camps for those left homeless by January's earthquake are functioning well.

"The real problem is in those rural areas, where there are not a lot of aid workers," she told CTV News Channel on Wednesday.

"I think they didn't predict the magnitude of what is unfolding here now."

Many of the aid workers who arrived in January to offer earthquake relief have now moved over to provide cholera relief.

The UN's Fisher said Tuesday that the international response has to be "significantly ratcheted up" as he appealed for more doctors and nurses to be sent.

Aid workers say the only way to truly get the outbreak under control is to prevent infection with the waterborne illness in the first place. But such a prospect is daunting, given that about half of Haiti's population doesn't have access to filtered water. Less than 10 per cent of the population has water piped into their homes.

At the Croix des Bouquets resettlement camp outside Port-au-Prince, workers are focusing on education to shore up the response against the epidemic.

"The level of awareness is really important for people to be able to manage their own health," said Jo-Ann Garnier-Lafontant, policy and advocacy director with plan Haiti, from.

The primary way of treating cholera is with rehydration. A litre of water with a spoonful of salt and a few spoonfuls of sugar is all that's contained in a rehydration kit.

"It's so simple what they need, yet without clean water, they can't solve this problem," LaFlamme said.

International organizations have been distributing water by truck to more than a million Haitians in hundreds of crowded refugee camps in the capital since January.

But LaFlamme said that moving around the capital is near impossible because cleanup from the quake is nowhere near finished.

"There are piles of rubble still everywhere 11 months after earthquake. And it just shows you how difficult it is for humanitarian aid workers to just get from A to B to bring the necessary medical equipment to start up these cholera treatment centres," she said.

With files from The Associated Press