Nigeria's biggest opposition party has gone to court challenging President Goodluck Jonathan's election victory and alleging electoral fraud.
The Congress for Progressive Change (CPC) wants elections to be rerun in several southern areas, which voted overwhelmingly for Mr Jonathan.
The CPC's Muhammadu Buhari gained 32% of the vote, while Mr Jonathan got 59%.
The announcement of these results sparked widespread unrest in northern areas which had backed Mr Buhari.
An estimated 500 people were killed, thousands of people forced from their homes and some mosques and churches set on fire.
The election threatened to reignite Nigeria's religious and ethnic tensions, with most voters in the largely Muslim north backing Mr Buhari, while those in the mainly Christian and animist south supported Mr Jonathan.
"[The] election should be seen to be transparent, free and fair," CPC chairman Tony Momoh said after filing the law suit in the capital, Abuja.
"We have detailed election malpractices in the south-south, south-east, some states in the south-west geopolitical zones and even some states in the north," he said.
"We want the tribunal to nullify elections in these areas where there were flaws and conduct fresh elections in those areas," he said.
Most observers said the elections were the best organised since the return of democracy in 1999.
Previous elections have been marred by widespread and blatant fraud, with armed thugs employed by local political bigwigs storming polling stations and filling in all the ballot papers.
Mr Buhari, a former military ruler, has said he was cheated of victory in two previous elections.
Mr Jonathan was appointed to the presidency last year upon the death of incumbent Umaru Yar'Adua, a northerner whom he had served as vice-president.
Many in the north felt the next president should have been from their region, as Mr Yar'Adua died before he could finish his term.
Mr Buhari won most of the mainly Muslim northern states but nationwide only gained half as many votes as President Jonathan.
Analysts say the violence has more to do with poverty and economic marginalisation in the north than religion.
The north and south also have cultural, ethnic and linguistic differences.
To win at the first round, a candidate not only needs the majority of votes cast, but at least 25% of the vote in two-thirds of Nigeria's 36 states. Goodluck Jonathan, of the PDP, reached that threshold in 31 states; runner-up Muhammadu Buhari of the CPC only did so in 16 states.
Nigeria's 160 million people are divided between numerous ethno-linguistic groups and also along religious lines. Broadly, the Hausa-Fulani people based in the north are mostly Muslims. The Yorubas of the south-west are divided between Muslims and Christians, while the Igbos of the south-east and neighbouring groups are mostly Christian or animist. The Middle Belt is home to hundreds of groups with different beliefs, and around Jos there are frequent clashes between Hausa-speaking Muslims and Christian members of the Berom community.
Despite its vast resources, Nigeria ranks among the most unequal countries in the world, according to the UN. The poverty in the north is in stark contrast to the more developed southern states. While in the oil-rich south-east, the residents of Delta and Akwa Ibom complain that all the wealth they generate flows up the pipeline to Abuja and Lagos.
Southern residents tend to have better access to healthcare, as reflected by the greater uptake of vaccines for polio, tuberculosis, tetanus and diphtheria. Some northern groups have in the past boycotted immunisation programmes, saying they are a Western plot to make Muslim women infertile. This led to a recurrence of polio, but the vaccinations have now resumed.
Female literacy is seen as the key to raising living standards for the next generation. For example, a newborn child is far likelier to survive if its mother is well-educated. In Nigeria we see a stark contrast between the mainly Muslim north and the Christian and animist south. In some northern states less than 5% of women can read and write, whereas in some Igbo areas more than 90% are literate.
Nigeria is Africa's biggest oil producer and among the biggest in the world but most of its people subsist on less than $2 a day. The oil is produced in the south-east and some militant groups there want to keep a greater share of the wealth which comes from under their feet. Attacks by militants on oil installations led to a sharp fall in Nigeria's output during the last decade. But in 2010, a government amnesty led thousands of fighters to lay down their weapons.