Zimbabwean opposition leader Nelson Chamisa has reportedly said that he "fears" the recent bombing at a Zanu-PF rally could be used to clamp down on opposition parties ahead of the country's crunch polls next month. According to Reuters, the Movement for Democratic Change leader says that the unexplained blast was likely going to be used by the government to clamp down on "certain individuals, certain candidates that they perceive to be their credible opposition".An AP report says there are 23 candidates standing for president in the July 30 elections, the first without former president Robert Mugabe since independence in 1980.The attack at the end of Mnangagwa's rally in Zimbabwe's second largest city, Bulawayo, last month killed two of the president's security officers and was the first serious violence in the current election campaign.Chamisa said he was confident of emerging victorious in the forthcoming elections and would also include some elements of the ruling Zanu-PF party in his government, Reuters reported.Chamisa's remarks came a day after President Emmerson Mnangagwa had reportedly said that a group linked to former first lady Grace Mugabe could be behind an explosion that rocked a stadium where he addressed his Zanu-PF supporters.Mnangagwa escaped unhurt, but at least 49 people were injured, including Zanu-PF chairperson Oppah Muchinguri-kashiri and Vice President Kembo Mohadi who were airlifted to South Africa for medical assistance. According to BBC, Mnangagwa said he suspected that a group called the Generation 40 (G40), which supported Grace for presidency, was responsible for the explosion.However, he said that investigations were still under way and he expected arrests to be made soon.According to News24, rights group have expressed concerns over the country's electoral violence ahead of the July 30 polls. Speaking to reporters in Johannesburg this week, Civil Society leaders from Zimbabwe said the elections in their country were going to take place in an "environment where the security of citizens is still a major issue"."There is a huge risk that we may not have a credible free and fair election in Zimbabwe on July 30," Human Rights Watch's Dewa Mavhinga told reporters.He said although Mnangagwa has made several assurances and pledged to deliver a fair and credible election, there were still major concerns ahead of the elections."He [Mnanganwa] is talking the right things [but] it is not clear whether he is walking the talk. The challenge which we have put to his administration is to deliver in terms of concrete steps to deliver credible and fair elections," he said.* Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and Facebook.