Zimbabwe's President Emmerson Mnangagwa has said he suspects a group linked to the former first lady was behind an attempt on his life.
Two people died and more than 40 were hurt in an explosion near Mr Mnangagwa at a rally in Bulawayo on Saturday.
Mr Mnangagwa told the BBC's Fergal Keane that he suspected the G40 group, which supported Grace Mugabe for the presidency, had carried out the attack.
Mrs Mugabe's husband, Robert Mugabe, was forced from power last year.
The army intervened to oppose Mrs Mugabe's attempt to succeed her husband as the country's leader, and the ruling Zanu-PF party then sacked Mr Mugabe, replacing him with Mr Mnangagwa.
An exiled member of the G40 group, former government minister Jonathan Moyo, earlier tweeted that the blast "smacks of an inside job".
Mr Mnangagwa did not accuse Mrs Mugabe of being involved in the attempt on his life.
He told the BBC he expects arrests to be made shortly.
"I don't know whether it was one individual - I would think it is broader than one person. I would think this is a political action by some aggrieved persons," he said.
'Soft as wool'
Mr Mnangagwa described Mrs Mugabe as someone who had frequently insulted him in the past.
"On what basis would I trust someone who was used by a cabal to say things that had no basis?" he asked.
Nicknamed "the crocodile", Mr Mnangagwa has a reputation for being ruthless, but he told our correspondent: "I am as soft as wool. I am a very soft person in life, my brother. I'm a family person. I am a Christian."
Despite the apparent attempt on his life, Mr Mnangagwa said there would be no countrywide security clampdown and elections scheduled for 30 July would go ahead in a free and fair manner.
Zimbabwe was stable and that foreign investors should not worry, he said.
Representatives of Zanu-PF, the main opposition Movement for Democratic Change (MDC) and other parties signed a peace pledge on Tuesday, promising to avoid violence and hate speech during campaigning.
The MDC's Nelson Chamisa is expected to be Mr Mnangagwa's main challenger in the election. He has also condemned the attack in Bulawayo.