Two Cameroonian opposition parties agreed a coalition on Friday to jointly contest this weekend's presidential polls, a spokesperson said, in an effort to dislodge six-term President Paul Biya.The announcement confirms rumours of an opposition manoeuver to try to defeat 85-year-old Biya at Sunday's polls, but the new union will not include the main opposition Social Democratic Front's hopeful Joshua Osih.Akere Muna, leader of the People's Development Front (FDP) agreed to "withdraw his candidacy... and to support Maurice Kamto," leader of the Movement for the Rebirth of Cameroon (MRC), Muna's spokesperson said in a statement.Kamto's party alleged earlier on Friday that a "massive fraud" was underway to ensure the sitting president wins a seventh term.Maurice Kamto's campaign chief Paul-Eric Kingue said efforts to rig Sunday's ballot "had the blessing" of the electoral commission."We won't accept any result if this kind of fraud continues," he said at a media briefing by Kamto's MRC in the capital Yaounde.Cameroon's authorities have long faced accusations of unfairly favouring Biya, one of Africa's longest serving rulers who won 78% of votes in 2011's polls which were described as "flawed" by observers and the opposition. 'Attempting to spread chaos' Cameroon will vote amid tight security including a ban on internal travel by road, rail and air from 1700 GMT Saturday until the conclusion of voting.The borders of the central African nation of 25 million people will also be sealed 48 hours ahead of voting day, according to a decree seen by AFP.Kamto's MRC has alleged that polling cards have been forged and that voter registration has continued despite the process being officially closed."In 62 percent of areas, the (ruling) Cameroon People's Democratic Movement party is still adding names to the voter roll," said Kingue."We're not preparing for war, but wherever there is fraud, there will be a firm response."Communication Minister Issa Tchiroma Bakary said in a statement that "measures have been taken... to prevent any actions that might create violence or disorder around the elections".He added that "political figures linked to foreign interests were preparing groups of agitators to stir-up violence in the event that the results don't go their way".The minister did not specify who he was referring to."By attempting to spread chaos, they risk being unpleasantly surprised," he said. The authorities would "not tolerate any disorder before, during or after the presidential vote," he added.Biya has been in power for 35 years and now faces seven opposition contenders following Muna's withdrawal. The opposition front-runners are Osih and Kamto.* Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and Facebook.