Accra - Tension was mounting in Ghana on Thursday over delays in releasing the results of a nail-biting presidential election already tainted by sporadic outbreaks of violence.The high-stakes race between incumbent John Mahama and rival Nana Akufo-Addo is seen as a litmus test of stability for one of Africa's most secure democracies.In its latest evening update, the country's electoral agency said that they had received preliminary results from 90 of out 275 constituencies, but only published the results for 25 of them. "Please be patient," Electoral Commission head Charlotte Osei said at a press conference, "accuracy is more important than speed".Following "possible instances of over-voting" the commission had previously said that the tallies would be subject to extra verification. But after doing their own calculations of the publically posted constituency results, local media organisations report that Akufo-Addo has taken the lead over Mahama.PeaceFM says that Akufo-Addo is in the lead with 53.96% of the vote calculated from just over half of the 275 constituencies. CitiFM also puts Akufo-Addo in the lead with 52.70% of the vote calculated from 104 constituencies. In the early afternoon, New Patriotic Party (NPP) supporters armed with golf clubs were already gathering outside Akufo-Addo's house preparing for a face-off. "We have won the election," said 33-year-old Bismark Agyei. "They need to announce that."NPP 'propaganda'Internal polling results released by Akufo-Addo's team heaped additional pressure on the commission.After analysing "pink sheets" - carbon copies of the voter tallies at individual stations - the NPP reported that Akufo-Addo is winning with a lead of over one million votes.NPP spokesperson Oboshie Sai Cofie said it was clear the party had won. "As more and more of the results come in, we'll reach a point of no return," Cofie said.NDC deputy general secretary Koku Anyidohu dismissed the NPP claim as "propaganda", adding that releasing the internal results was "illegal" and designed to "hoodwink" the press. "We're all allowed to do our pie charts and bar graphs, but they are playing you."Still, electoral observers are urging the commission to "speed up the process of announcing results," said Christopher Fomunyoh, regional director for the US-based National Democratic Institute. "Then it assures people that the process is working as planned," Fomunyoh said.Be 'vigilant' Akufo-Addo had told his supporters that "vigilance is key" at the polls in an attempt to avoid a repeat of the 2012 vote -- narrowly won by Mahama with 50.7% - that he contested unsuccessfully in the country's Supreme Court."We wish to state emphatically that the NPP will not allow any person, persons or entity to subvert the will of the Ghanaian people and that all necessary steps will be taken to protect the vote," party campaign chairman Peter Mac Manu said in a statement just after midnight. After voting Wednesday, Mahama voiced confidence the election would "consolidate that democracy further".There are seven candidates battling for the top job - including former first lady Nana Konadu Agyeman-Rawlings - and if the smaller parties perform well and deny either frontrunner a majority, a run-off vote will be held later this month.Charismatic Mahama, 58, is running for a second term. The leader of the ruling New Democratic Congress (NDC) party has urged voters to "stay the course", promising to deliver more infrastructure projects. Akufo-Addo, 72, is making his third and likely final bid for highest office. He has blasted Ghana's poor economic growth estimated at 3.3% in 2016 - the slowest rate in two decades - and outlined how to get the economy back on track. Akufo-Addo, who said previously that he would accept the results even if he loses, added recently: "Those are hurdles we have to jump once we get there".