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.