Health Minister David Parirenyatwa has said people should stop shaking hands to prevent the disease spreading.
He said it was the worst ever cholera outbreak in Harare.
Cholera can be treated easily but hospitals lack medicines and staff.
The WHO said Zimbabwe's health facilities face a "massive gap" in required medicines due to a drop in local manufacturing capacity, weakened in turn by a shortage of foreign currency.
Marcus Bachmann from medical charity Medecins Sans Frontieres told the BBC that many poorer areas of the capital had been without water for several months and this was a major factor in the severity of the outbreak.
"Proper hygiene is the best protection against cholera and you can't do that without clean water," he said.
Cases of cholera have been reported either side of Zimbabwe's borders with South Africa, Botswana and Mozambique, showing the sub-regional threat of the outbreak, it said.
The BBC's Peter Biles reports from the South African town of Musina, near the border with Zimbabwe, that cholera patients are being treated at an emergency centre on the lawn in front of the hospital.
Emergency cholera centre in South African town of Musina
One cholera victim from Harare told him that on Zimbabwe's side of the border, toilets had not functioned for one month, and people were "defecating everywhere".
"We need the world to help us. The country is dying and people are dying."
South Africa's ministry of health has confirmed more than 160 cholera cases, including three deaths.
Zimbabwe's government has blamed its crisis on Western sanctions it says are aimed at trying to bring down President Robert Mugabe.
But the sanctions imposed after allegations of electoral fraud and political violence are aimed at Mr Mugabe and his close associates and consist of travel bans and a freeze on their foreign assets.
Zimbabwe is facing a severe economic crisis.
The latest estimated annual inflation rate was 231,000,000%, and just one adult in ten is thought to have a regular job.
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