Attempts on Emmerson Mnangagwa's life have been made before, so much so that after Saturday's blast the president said he was "used to" it.

Zimbabwe's President Emmerson Mnangagwa narrowly escaped injury when a blast exploded at a political rally on Saturday: something "exploded a few inches away from me, but it is not my time," he said.

Although Mnangagwa escaped harm, Vice President Kembo Mohadi, his wife Constantino Chieqnga and several government ministers were injured. Two people have been reported dead and about 150 injured, after the blast. Health Minister David Parirenyatwa confirmed that at least 41 people have been severely injured.

"These are my mortal enemies and the attempts have been so many. It's not the first attempt [on] my life. I'm used to it. Six times my office has been broken into; cyanide was put in my offices so many times. I will continue," Mnangagwa remarked.

The blast came as Zimbabwe prepared to hold its first post-Mugabe presidential election on July 30, with 75-year-old Mnangagwa and 40-year-old Nelson Chamisa, the leader of the opposition Movement for Democratic Change, the main contenders.

The rally was Mnangagwa's first in Bulawayo, an opposition stronghold where the ZANU-PF party of former President Robert Mugabe and now Mnangagwa has not won a national election since 2000.

The cause of the blast is unknown and there have been no immediate claims of responsibility. The pre-election season had previously been peaceful.

"The campaign so far has been conducted in a free and peaceful environment, and we will not allow this cowardly act to get in our way as we move towards elections," Mnangagwa said. "It's just an element of defeatists in the struggle of freedom. The country is peaceful."

Opposition leader Nelson Chamisa expressed sympathy to the victims of the attack and called for a thorough police investigation. "Political violence of any nature from any quarter is totally unacceptable," Chamisa said.

Attempts on Mnangagwa's life have been made before, so much so that after Saturday's blast the president said he was "used to" it. At a rally last August, when Mugabe was still president, Mnangagwa was poisoned.