Nigeria election: President Goodluck Jonathan accuses
Nigerian President Goodluck Jonathan has said that the violence in the country that followed his re-election "was not a spontaneous reaction".
Tens of thousands of people have fled their homes because of the violence, says the Red Cross.
"I don't want to accuse anybody but we believe that people must be behind this," Mr Jonathan told CNN.
Poll runner-up Muhammadu Buhari denies instigating the "sad, unfortunate and totally unwarranted" events.
Nigeria is divided by rivalry between the predominantly Muslim north and the mainly Christian south, which also have cultural, ethnic and linguistic differences - so much so that the presidency has often alternated between people who come from each of the two halves of the country, in an attempt to keep the peace.
Riots broke out in the north after Mr Jonathan, a southerner, emerged as the winner of the presidential poll. A civil rights group says the unrest has left more than 200 dead, while hundreds of arrests have been made.
Gen Buhari has said the Nigerian election commission's computers were programmed to disadvantage his party in some parts of Nigeria.
But he urged his supporters to refrain from attacks, saying: "It is wrong for you to allow miscreants to infiltrate your ranks and perpetrate such dastardly acts as the mindless destruction of worship places.
"Needless to say, this act is worse than the rigging of the elections."
Umar Marigar of the Red Cross told the BBC on Wednesday that the number of displaced people had trebled 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 because they feared reprisal attacks against northerners.
He added: ''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."
'Free and fair'
Shehu Sani, head of the Civil Rights Congress, told the AFP news agency: "In the whole region, from reports reaching Civil Rights Congress, the death toll is over 200."
He added that more than 1,000 people had been arrested in the city of Kaduna alone.
The BBC's Abdullahi Kaura Abubakar says Kaduna city is now calm, with markets open and people shopping for food.
The security forces are patrolling the streets and police helicopters can be heard flying overhead intermittently.
The streets of the city were left littered with charred corpses after rioters burned churches, police stations and homes during two days of disturbances.
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.
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, a northern Muslim 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.