Nigeria election: Buhari party to challenge results
The runner-up in Nigeria's presidential poll, Gen Muhammadu Buhari, has said there were widespread irregularities in Saturday's election.
There were areas in the south where his supporters were not allowed to vote, the ex-military leader told US radio.
But he said his party would challenge the result legally and urged calm after riots broke out in the north when Goodluck Jonathan, a southerner, won.
The Red Cross says 48,000 people have now fled from the violence.
In Kaduna state, police sources have told the BBC some 400 people have been arrested in connection with the clashes.
The BBC's Abdullahi Kaura Abubakar says it is calm now in Kaduna city, where streets have been left littered with burnt corpses and rioters burned churches, police stations and homes burnt after two days of disturbances.
There are clashes in other parts of the state and more security forces have been deployed to those areas, he says.
Mr Jonathan was declared winner of Saturday's presidential poll, with the electoral commission saying he received about 57% of the vote with 22.5 million votes to General Buhari's 12.2 million votes.
International observers have said the election was reasonably free and fair.
Gen Buhari told Voice of America's Hausa-language radio service that his Congress for Progressive Change party had noticed irregularities in the south and south-east of the country.
"I urge people to calm down and be law-abiding as we are pursuing these irregularities with [the electoral commission] with a view to ensure justice for them," he said.
Umar Marigar, the Red Cross's national disaster co-ordinator, told the BBC on Wednesday that the number of displaced had tripled in the last day - from 16,000 to 48,000, mainly in the north.
But he said that in the southern state of Anambra, 8,400 people had sought refuge at the Onitsha military barracks, fearing reprisal attacks against northerners.
Mr Marigar said he was concerned that the political violence was becoming a clash between ethnic and religious groups.
''The violent protests turn from political into ethno-religious crisis. As such, people might like to engage in retaliatory attacks. This is what we are always afraid of. That's why we don't quote the figures of dead people," he said.
The BBC's Abdul Mohammed Isa in Port Harcourt, the main city in the southern Niger Delta, says it is calm, with police and military patrolling the streets as usual.
Mr Jonathan, a Christian from the oil-producing Niger Delta, was appointed to the presidency last year upon the death of incumbent Umaru Yar'Adua, whom he had served as vice-president.
He staked his reputation on the election, repeatedly promising it would be free and fair.
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.