Nigeria's governing party is to choose its candidate for presidential elections due to be held in April. The strongest challenge to President Goodluck Jonathan is expected to come from ex-vice-president Atiku Abubakar. The PDP candidate has won every poll since the end of military rule in 1999, so its candidate will be favourite. But some analysts say the PDP could split over the vote with southerners backing Mr Jonathan and Mr Abubakar supported by those from the north. The PDP has a tradition of alternating power between north and south after two terms of office but this was interrupted when Mr Jonathan, a southerner, succeeded Umaru Yar'Adua when he died last year. There is tight security in the capital, Abuja, where the voting is taking place, following recent bomb attacks. Some 5,000 PDP delegates have gathered in the city's Eagle Square where the vote is to take place. The square was hit by twin car bombings last October, as Nigeria celebrated 50 years of independence. Veteran politician Sarah Jibril is also seeking the PDP ticket but the BBC's Tomi Oladipo says she has sought the party's nomination in past elections with no success and is not expected to pose a threat this time either. Last year, a group of Nigeria's powerful governors said they would back President Jonathan's candidacy but only if he stood for one term before standing down in favour of a northerner. He is the first president from Nigeria's southern, oil-producing Delta region. But several northern powerbrokers have backed the campaign of Mr Abubakar. Nigeria's recent elections have been tarnished by fraud and violence. Mr Jonathan has promised to introduce electoral reforms, but correspondents say it will be difficult to implement radical changes before April. The main opposition candidates are former anti-corruption campaigner Nuhu Ribadu and Gen Muhammadu Buhari. Source: BBC