More than 200 people have been killed and an estimated 40,000 others displaced in violence triggered by news of Goodluck Jonathan's victory in the Nigerian presidential election.
On Wednesday hundreds of men allegedly involved in the deadly violence were rounded up and taken to a police station in Kaduna in northern Nigeria.
Officials have arrested more than 300 people in an attempt to restore order, and a curfew that had been in place was being lifted until dusk.
A Nigerian civil rights group based in Kaduna put the number of deaths at more than 200 across the north, after rioting broke out on Sunday.
"In the whole region, from reports reaching Civil Rights Congress, the death toll is over 200," Shehu Sani, the head of the organisation, told AFP news agency.
There were reports of fresh clashes in an area of the state of Kaduna overnight, with a community leader telling local radio "the killing was unbelievable and the destruction is colossal".
Rioting erupted in major northern cities after incumbent Jonathan was declared the winner with 57 per cent of the ballots, easily beating his rival, Muhammadu Buhari, with 31 per cent.