Posted by BBC News on
Nigerians go to the polls on Saturday to elect a new president and National Assembly members in the second round of elections.
This is the first time since independence in 1960 that power is being transferred from one civilian president to another in Africa's most populous nation.
Opposition parties and observer groups have called for the vote to be postponed. The opposition has threatened a boycott unless the electoral commission is disbanded and other conditions to ensure transparency are met.
What happened during the first round of voting?
The first round of voting was held last Saturday for regional assemblies.
Voting was delayed by four hours and several policemen were killed when violence erupted in the Niger Delta during the polling.
Three police stations were burnt down in Port Harcourt city and vans transporting election material were also set ablaze.
The main opposition groups and local election observers said the results announced in 10 of Nigeria's 36 states did not reflect the will of the people and should be rejected. The EU also criticised the poll and said re-runs should be considered in several states.
They said that most of the electoral staff were from the ruling PDP and that illegal polling stations had been set up where all the votes had been cast for the ruling party.
How does the system work?
The president and 36 state governors are elected directly for a maximum of two four-year terms. President Olusegun Obasanjo has already served his maximum term and is not in the presidential race.
There are 24 presidential candidates in this year's election, in which 46 registered political parties are participating.
To win, a candidate must receive the highest number of votes and at least one quarter of the votes in 24 of the 36 states.
If there is no clear winner, there will be a run-off between the candidate with the highest number of votes and one of the candidates who holds the majority votes in the most states.
Presidential election results are expected no sooner than 48 hours after polls close, according to Nigerian radio.
Who are the main candidates?
1. Umaru Musa Yar'Adua - People's Democratic Party (PDP). Umaru Musa Yar'Adua, the governor of Katsina State, is the candidate for the ruling PDP of outgoing President Obasanjo. Despite losing some founding members to the opposition, the party remains popular across Nigeria.
2. Gen. Muhammadu Buhari - All Nigerian People's Party (ANPP). Gen Buhari is a former military ruler who led Nigeria from January 1984 until August 1985 when he was deposed in a coup. He is the candidate for the ANPP party, which is most popular in the mainly-Muslim north of the country but also enjoys nationwide support. It was the strongest challenger to the PDP in the 2003 elections.
3. Atiku Abubakar - Action Congress (AC). Atiku Abubakar was President Obasanjo's deputy from 1999 until August 2006 when the president accused him of corruption and had him investigated. He has denied the charges, and the court battle continues to cast doubt on his right to stand in the election. He is running under the umbrella of the Action Congress, which was formed by dissenting PDP leaders and opposition politicians and draws support from across the country.
4. Orji Uzor Kalu - Progressive Peoples Alliance (PPA). Orji Uzor Kalu, the multi-millionaire governor of Abia State, is the presidential candidate for the Progressive Peoples Alliance. PPA is a new party formed by PDP dissenters in southeastern Nigeria.
5. Chukwuemeka Odumegwu Ojukwu - All Progressives Grand Alliance (APGA). Chukwuemeka Odumegwu Ojukwu, led the failed Biafra secessionist movement in 1967 which waged a three-year civil war in Nigeria. His party, the APGA, has a strong base in the south east.
6. Prof. Patrick Utomi - African Democratic Congress (ADC). Prof. Patrick Utomi's ADC party is composed mainly of younger-generation intellectuals. It has a strong appeal among Nigerians in the diaspora.
What are the main poll issues?
1. Violence - Many Nigerians fear further violence will take place during the polls. Violence and human-rights abuses were described as the "hallmark of Nigeria elections" in a joint report by Amnesty International and Nigerian organizations. Officials say they have taken measures to combat election violence.
2. Security - There is deteriorating security situation in the Niger Delta - Nigeria's economic powerhouse. The region's oil and gas riches provide about 90 per cent of the country's foreign earnings. But militant youths, actively supported by the local people, have turned the entire mangrove creeks into a vast minefield. In February, the Movement for the Emancipation of the Niger Delta (Mend), an umbrella body of the major militant groups, issued a public statement threatening war.
3. Poverty - There is concern over the scale of poverty across Nigeria, despite its position as Africa's leading oil producer.
Are there any foreign observers?
The Economic Community of West African States and the EU are monitoring the election. The Christian Association of Nigeria and Nigeria's Supreme Council on Islamic Affairs have also made 240,000 observers available.