Accra - A supporter of Ghana's main opposition party was killed this week following violent clashes at a campaign rally as tensions flared ahead of presidential polls, police said on Tuesday.Six people were in a critical condition following Monday's clashes which were the latest violence in the run-up to Wednesday's presidential vote. Incumbent President John Mahama, who has appealed for calm, will face off with opposition favourite Nana Akufo-Addo.Last month Akufo-Addo's Accra residence was reportedly attacked while he was away on a campaign tour in what his party claimed was an attempt to "create an atmosphere of fear within Ghana".Ken Yeboah, deputy police commissioner for the northern region, said a supporter of Akufo-Addo's NPP party was beaten to death during a political rally in Chereponi on Monday at which clashes erupted between supporters of the president and backers of the opposition. "There was a clash between the youth... they started throwing stones, fighting, then the police went in and separated (them)," Yeboah said. He said that despite the police intervention, clashes resumed and the youths began to set fire to campaign materials and discharge firearms. "The young person who died was beaten," he said, adding that 14 other supporters were injured in the violence, including the six who are in a critical condition.Yeboah described the clashes as "disappointing" and appealed for calm. "For now both parties can be blamed for it. We can't blame security," he said.Macho men Tensions have been rising in the run-up to the election in Ghana, which is normally seen as a beacon of stability and democracy in Africa.The problems have been exacerbated by hulking vigilantes known as "macho men", reportedly hired by political parties to protect politicians.But they have been accused of going beyond their brief."They pose a real risk in terms of violence, intimidation and creating the basis for violent acts with the possibility of reciprocal responses," said Dr Kwasi Aning, director of the Faculty of Academic Affairs and Research at the Kofi Annan International Peacekeeping Training Centre (KAIPTC).The centre has this year identified 16 militias in Ghana, with eleven based in Tamale -- the region rocked by Monday's violence.With names like "Aluta (wrestling) Boys", "Pentagon", "Al Qaeda" or "Al Jazeera", members are mostly illiterate and tend to come mostly from poor backgrounds in the Muslim-majority region.The main political parties have denounced the groups, but Aning remains sceptical."That is the rhetoric (but) political parties have not really distanced themselves. There have been meetings between the police and these groups in which assurances have been given to the police, but the reality on the ground is different," he said.The militias have been accused of stealing ballot boxes and attacking or intimidating their political opponents in Ghana's past elections.The flagging economy and corruption allegations against officials have also been key issues during campaigning.With exports of gold, cocoa and oil, Ghana's economy enjoyed significant growth until recent years when huge foreign debts slowed down progress. Another issue that may affect voters is technical problems at polling stations which government worker Abdul Basit believes may prevent people from casting their ballots. "That could bring violence and tension. That is our fear. Especially for this part of the country (Tamale)," said Basit.Following the last elections in 2012, Akufo-Addo - who polled 47.7% of the vote compared to Mahama's 50.7% - contested the results in Ghana's constitutional court, although he was ultimately unsuccessful. This time he has vowed to accept the results even if he loses.