HARARE-- Zimbabwe ruled out delaying July elections after a bomb blast at a ruling party rally in which President Emmerson Mnangagwa narrowly escaped unharmed.

Vice President Constantino Chiwenga -- who was among those injured in Saturday's attack -- said the first parliamentary and presidential polls of the post-Robert Mugabe era would go ahead as scheduled on July 30.

"Let me be very clear, nothing will stop the election in Zimbabwe, nothing at all," Chiwenga told a rally of the ruling ZANU-PF party in the town of Chitungwiza near Harare.

"That act of terrorism in Bulawayo is nothing. It does not deter anyone," he said, adding that no political rallies would be cancelled and urging fellow politicians to "continue to campaign peacefully".

Footage from Saturday's event showed a device exploding and plumes of smoke billowing around Mnangagwa as he descended from the podium at the White City stadium in Zimbabwe's second largest city Bulawayo.

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.

Police said 49 people, including Chiwenga and fellow Vice President Kembo Mohadi, were injured, some seriously, and that investigations were ongoing.

It is not known who carried out the attack, and there has been no claim of responsibility.

While Bulawayo has long been a bastion of opposition to the ZANU-PF and it was Mnangagwa's first rally in the city, commentators suggest the attack could have been sparked by internal party ructions.

Mnangagwa, 75, said the device "exploded a few inches away from me".

"These are my mortal enemies and the attempts have been so many," he told the state broadcaster on Saturday night.

"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."

July's polls will be the first ballot box test for Mnangagwa, and the first since Mugabe resigned following a brief military takeover in November after 37 years in power.

Mnangagwa -- Mugabe's onetime right-hand man who was known as the "Crocodile" for his ruthlessness -- called Sunday for unity and peace.

"In November, we all came together motivated by a dream, (for) a free, democratic and prosperous Zimbabwe, a peaceful Zimbabwe," he said in an audio clip.

"Now some people are trying to kill our dream.

"While we have all chosen the path of peace others unfortunately still cling to the tools of violence. I assure you they will not succeed," he said.

"We as a people must unite."

Previous elections in Zimbabwe had been marred by electoral fraud, intimidation and violence, including the killing of scores of opposition supporters in 2008.

This time around, Mnangagwa has pledged to hold free and fair elections as he seeks to mend international relations and have sanctions against Zimbabwe dropped.

Zimbabwe police spokeswoman Charity Charamba said "comprehensive investigations are in progress" into the blast.

Plain-clothes police conducted random stop-and-search operations on the streets of Bulawayo while armed police mounted check points along the main highway leading out of the city.