Zimbabwe's police on Wednesday issued a ban on all public gatherings to control the spread of cholera, which has killed 21 people in the capital Harare, a day after the government declared an emergency following the outbreak of the disease.

In a statement, the national police spokesperson Charity Charamba urged members of the public to take heed of the ban as it would assist in alleviating the spread of cholera, but did not say how long the ban would last.

The main opposition leader Nelson Chamisa had planned to hold a rally on Saturday where he may take a mock presidential oath, three weeks after President Emmerson Mnangagwa was inaugurated following a court decision upholding his disputed election victory.

Chamisa's spokesperson Nkululeko Sibanda could not comment immediately.

The opposition leader visited clinics where cholera patients were being treated earlier on Wednesday and called for collaboration between city health officials and the government.

The Minister of Health Obadiah Moyo told a meeting of government departments dealing with the outbreak that more than 3 000 people had been infected by cholera and the disease had now spread outside the capital.

This is the biggest cholera outbreak since 2008 when 4 000 people died and more than 40 000 were treated for this disease, according to ministry of health data.