President Goodluck Jonathan has a strong lead in Nigeria's presidential election, taking almost twice the number of votes as his main rival as counting continues. Results from 30 of 36 states showed the incumbent had won 20, while ex-military leader Muhammadu Buhari had nine. Tens of millions of votes were cast in Saturday's poll. Mr Jonathan has staked his reputation on the the election, repeatedly promising it would be free and fair. If current trends continue, President Jonathan - a Christian from the oil-producing Niger Delta - will avoid the need for an election run-off, says the BBC's Caroline Duffield in the Nigerian capital Abuja. 'New dawn' International monitors said Saturday's election could be Nigeria's first credible vote for decades. Previous polls were marred by widespread violence and vote-fixing. Counting began as soon as polls closed on Saturday, and Mr Jonathan quickly led the field in the south of Nigeria - he hails from the oil-rich Niger Delta region - while Gen Buhari was seen to be mounting a strong challenge in his heartland - the mainly Muslim north. To win at the first round a candidate needs at least 25% of the vote in two-thirds of Nigeria's 36 states. Official results are expected on Monday. Voting was reported to have generally gone smoothly, despite some reports of fraud and incidents of violence. Some violence was reported on polling day, with a woman said to have been killed in the central city of Jos and two bomb explosions in the north-eastern city of Maiduguri. Dozens of people were killed in the run-up to the vote. Mr Jonathan cast his ballot in his home state of Bayelsa in the oil-rich Niger Delta. After voting, he said the election was a "new dawn in Nigeria's political evolution". "If the ballot paper means nothing then there is no democracy… Nigeria is now experiencing true democracy where we the politicians have to go to the people," he said. He said he was confident of victory, and said he would leave office if he lost. Other challengers for the presidency include former anti-corruption chief Nuhu Ribadu and Kano state governor Ibrahim Shekarau, though both are seen as rank outsiders. In Daura, home to Gen Buhari, crowds waited for hours despite the intense heat to cast their votes. He insisted the election was not as clean as Mr Jonathan was suggesting. "There's a desperate attempt by the ruling party to rig this election in a more sophisticated way," Gen Buhari told the BBC. "This time around - the level of awareness and commitment by the masses is what has given me some relief." Gen Buhari added that he had more faith in the Independent National Electoral Commission (Inec) this time round, adding that "probably 60% of the election will be credible". Chequered past Africa's largest oil producer has long been plagued by corruption and has a history of vote fraud and violence. Allegations of ballot-stuffing plagued the 2007 election, which brought Mr Jonathan to power as the vice-president. He took over as president in 2010 when the incumbent died, becoming the first leader from the oil-producing Niger Delta region. Mr Jonathan's People's Democratic Party lost seats in a parliamentary election last week. But he remains favourite in opinion polls, and his chances have been boosted after Mr Buhari and Mr Ribadu failed to agree a formal alliance to run against him. The relatively successful conduct of the parliamentary election has increased confidence in the ability of the electoral commission, Inec, to ensure a fair presidential vote. With 74 million registered voters, Nigeria has the biggest electorate on the continent. Source: BBC