Chamisa toured a health facility treating people and called on the United Nations to help contain the situation.
"It's more than just an emergency it is a national disaster," he said.
Britain warned prospective travellers to Harare about the cholera outbreak and urged travellers to learn to recognise symptoms of the disease.
Cholera outbreaks have occurred regularly in the city as authorities struggle to provide potable water and sanitation facilities.
Informal housing areas without running water have mushroomed and basic infrastructure has collapsed due to years of neglect.
Tests on water samples from some wells and boreholes showed the water was contaminated with cholera and typhoid-causing bacteria.
Zimbabwe, which was ruled by Robert Mugabe from independence in 1980 until his ousting last year, suffered its worst cholera outbreak in 2008.
A total of 4,000 people died and at least 100,000 people fell ill.
President Emmerson Mnangagwa, who succeeded Mugabe, has pledged to revive the economy and improve public services.
"To contain the outbreak and mobilise resources we have declared a state of emergency in Harare, and are working closely with our international partners," Mnangagwa said Wednesday on Twitter.
Unicef advised Zimbabweans to prevent the spread of cholera by regular hand-washing, drinking only safe water, washing food, cooking it thoroughly, and avoiding shaking hands.