"This is now a police issue but the president is safe at Bulawayo State House", spokesman George Charamba told Reuters.
Zimbabwe's president narrowly survived an attempt on his life this afternoon after a blast at a party rally.
Bulawayo is Zimbabwe's second-largest city and is traditionally an opposition stronghold.
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Mnangagwa suggested he was the target of the attack, which he said also injured Vice-President Kemo Mohadi.
Marry Chiwenga, the wife of Zimbabwe's First Vice-President, Constantino Chiwenga, was also injured and pictures on social media showed the president visiting her in hospital.
Mr Mnangagwa, who was sacked as Mugabe's deputy in a ruling party feud shortly before the transition, took over with pledges to deliver free and fair elections.
The President himself has openly joked about the attempts, including during his campaigning.
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Home Affairs Minister Obert Mpofu confirmed the "unfortunate incident" adding: "There were some injuries but I am still to get full details but as you are aware this happened close to the (president)".
State television immediately cut its broadcast. There have been frequent allegations of assassination attempts directed at Mnangagwa and other senior politicians, though none of these have involved bombings or firearms.
Those policies were cornerstones of Mugabe's near four-decade rule, but the ruling ZANU-PF says Zimbabwe is at a critical stage of its transition and needs an experienced hand like Mnangagwa at the tiller.
The July 30 vote is the first without Mugabe since independence in 1980. It was Mnangagwa's first rally in the city. Political violence in any form is unacceptable and contrary to the positive progress required to move Zimbabwe forward as it seeks to take its place on the global stage.
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