A record 23 people have filed to run for president in the July 30 election.
"As a long standing friend of Zimbabwe, Kenya sends its message of goodwill to President Emerson Mnangagwa and the people of the Republic of Zimbabwe, and urges restraint during the campaign and electioneering period because political competition should not fracture Zimbabweans but should be a measure to deepen their democratic credentials", the President notes.
Mnangagwa told state broadcaster ZBC, without elaborating, that those responsible must have come from "outside Bulawayo".
Mnangagwa, who was rushed away from the scene of the explosion, later visited the injured in hospital.
But Mnangagwa insisted that the "country is peaceful" as Zimbabwe prepares to stage its first ever elections not to feature former president Robert Mugabe on July 30. Health Minister David Parirenyatwa said some of the injured had serious stomach wounds.
Mnangagwa - who took power after Mugabe's ouster in November - said he was the target of the attack, which the state media is describing as an assassination attempt.
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Police are offering a reward for information leading to the arrest of suspects. "In the past 38 years political violence has been a permanent feature and an anticipated ritual ... which we must expunge", Chamisa said.
Chamisa condemned the attack, tweeting that "violence must have no place in our politics".
Zimbabwe's polls will be held in the presence of Western observers for the first time since 2002.
She said comprehensive investigations are underway on the blast which occurred shortly after Zimbabwean President Emmerson Mnangagwa had finished addressing the rally.
Both main candidates are campaigning on a pledge to revive an economy crippled by a legacy of often violent seizures of land from white commercial farmers and a black economic empowerment drive that targeted foreign-owned businesses.
Mnangagwa adopted most of Mugabe's security and bureaucratic system, fearing a complete overhaul, which the military had sought, could cause instability in government and cost him votes.