Zimbabwe's capital was under tight security on Wednesday as the Constitutional Court began to hear the main opposition party's challenge to the results of last month's historic presidential election.Police barricaded the streets around the court in central Harare amid high tensions over the crucial case which will decide if President Emmerson Mnangagwa's election victory is valid.The opposition claims the vote had "gross mathematical errors" and it seeks a fresh election or a declaration that its candidate Nelson Chamisa is the winner of the July 30 vote.The Zimbabwe Electoral Commission declared Mnangagwa narrowly won with 50.8% of the vote, avoiding a runoff. Chamisa received 44.3%, the commission said.* To follow the latest on the Zimbabwe elections SIGN UP TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and FacebookThe election was the first after the fall of longtime leader Robert Mugabe, who stepped down under military pressure in November. Many hoped the peaceful vote would launch a new era for Zimbabwe but two days later six people were killed when the military swept into the capital to disperse opposition protesters.Western election observers and diplomats condemned the "excessive" use of force. European Union election observers were in court on Wednesday; Mnangagwa badly needs a credible electoral process as a key step in removing international sanctions.Chamisa's challenge claims the electoral commission bumped up Mnangagwa's figures through double counts and the creation of "ghost" polling stations. It also alleges that some polling stations recorded more voters than those registered to vote."It's like a kid was playing with the figures," a lawyer for the opposition, Thabani Mpofu, told the court. He alleged that 16 polling stations had identical results and that "massive doctoring" took place.Mpofu told the court the electoral commission had produced three sets of presidential vote results, including one in court papers where the commission revised downwards Mnangagwa's win down to 50.67%. The commission attributed that to an "error" but argued it was not significant enough to invalidate the win.Mpofu said that in all Chamisa could have lost more than 69 000 votes, well over the 31 000 votes that allowed Mnangagwa to avoid a runoff election."On that basis a runoff is unavoidable," Mpofu said.Chief Justice Luke Malaba, however, pressed the opposition for the original election results forms to back up their allegations: "We cannot act on generalities."Mnangagwa and the electoral commission argue the opposition's application should be dismissed on a technicality, saying it was filed too late and that papers were not properly served on respondents.In his affidavit, Mnangagwa argues the court should not hear Chamisa'a application because he "scandalised" the court by claiming during political rallies that the judiciary was biased toward the ruling party, Zanu-PF.The case was being televised live by the state broadcaster, the Zimbabwe Broadcasting Corporation, but the courts ruled that the proceedings could not be livestreamed on social media. Journalists and others accredited by the court were following proceedings from a giant television screen on the court premises, but they were not permitted to carry mobile phones or laptops.Chamisa's lawyers in court said they had not been allowed to bring in electronic gadgets, either.According to Veritas, a legal think tank based in Harare, the court can declare a winner or invalidate the election and call for a fresh election or make any other order it considers "just and appropriate."If the court upholds Mnangagwa's win the inauguration would take place within 48 hours.This is not the first time the opposition has challenged election results in court. Following the 2013 presidential election, then main opposition leader Morgan Tsvangirai filed a challenge but later withdrew it, claiming he would not get a fair hearing. The court declined his withdrawal and proceeded to rule on the case in favour of Mugabe.