The Nigerian Red Cross say rioting in cities across northern Nigeria has killed many people with homes, churches and mosques set ablaze.
Police say the violence is being orchestrated by groups who refuse to accept the outcome of presidential elections at the weekend and is neither ethnic nor religiously-motivated.
Incumbent President Goodluck Jonathan, a Christian from the Niger Delta, has been officially declared the winner in the election, with 57% of the votes.
His main rival, former General Muhammadu Buharai - who is from the north - who received 31% of the vote.
The African Union earlier said it was Nigeria's best poll for decades.
However, the BBC reports that homes of Mr Jonathan's supporters were attacked in the cities of Kano and Kaduna.
Mr Jonathan 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.
In Kano, homes displaying posters of Mr Jonathan were set on fire, and gangs of young men roamed the streets shouting ''Only Buhari!''
A 24-hour curfew has been declared in Kaduna where TV stations are reporting that the home of Mr Jonathan's running mate, Vice-President Namadi Sambo, has been set on fire.
The city's central prison has reportedly been attacked and inmates released.
Mr Jonathan has appealed for an end to the violence.
Polling
Mr Jonathan has polled more than 22 million votes, compared with 12 million or so for General Buhari.
To win the first round, he needed at least 25% of the vote in two-thirds of the 36 states as well as a majority of the total votes cast.
According to regional results, Mr Jonathan passed that threshold in at least 24 states.
General Buhari has said he could not accept the results.
