Nigeria stepped up security on Saturday after a bomb blast in a northern city as the country geared up for the last leg in a series of elections marred by deadly post-poll violence.
A local rights group said nearly 250 people have been killed this week after riots broke out in northern Nigeria following President Goodluck Jonathan's victory in last weekend's vote.
Officials have so far kept the death toll under wraps but said they were stepping up security ahead of next Tuesday's state governorship elections.
"We are deploying more of our men in the troubled areas. We have noted the flash points and we are going to take particular attention to those. We will be more alert after the election," national police spokesman Olusola Amore said.
A man was killed overnight in Kaduna city when a bomb he was assembling exploded. Three of his colleagues survived with serious injuries, Aminu Lawal, spokesman for Kaduna police, said.
Many people have died in the predominately Muslim north in rioting after Jonathan, a southern Christian, won April 16 presidential polls.
A curfew is in effect in Kaduna state, where most of the deaths have been reported in the post-election violence.
The presidential vote exposed deep regional divisions in Nigeria, whose 150 million population is about half Christian and half Muslim belonging to some 250 ethnic groups.
Governorship and state assembly elections due on Tuesday have been postponed in Kaduna and the neighbouring state of Bauchi due to security concerns.
One man was killed and three others were wounded in the blast at a house in a predominately Muslim area of Kaduna city, the capital of the state of the same name, police said.
"One among the people in possession of the bomb lost his life and we arrested three people. They were the ones preparing the bomb," said Kaduna state police spokesman Aminu Lawal.
"Our security personnel are there still combing the place in case there are other devices," Lawal added by telephone.
Military patrols and curfews have largely restored calm, but an estimated 74,000 people have been displaced according to the Red Cross. Many are seeking refuge in police and military barracks.
The violence broke out as Jonathan defeated his northern rival, ex-military ruler Muhammadu Buhari, who has alleged rigging but has dissociated himself from the violence.
Nigeria's mainly Muslim north has long been economically marginalised compared to the oil-rich south, which is predominantly Christian.
Such violence in Nigeria is rarely strictly about religion and instead involves those who see themselves as the natives of an area attacking those viewed as the outsiders, even though they have in many cases been there for generations.
Jonathan said this week's violence was a reminder of the events that led to the country's 1967-1970 civil war, calling it "more than mere political protests."
Politicians have often stoked the bloodshed in the struggle for local power. Observers fear next week's elections for powerful governors - some of whom control huge budgets - could turn out to be more deadly.
The International Criminal Court prosecutor meanwhile announced he was probing this week's violence to determine whether it had been planned or organised.
The riots occurred despite what many observers have described as a reasonably well-organised election, marking a major step forward for a country with a history of deeply flawed ballots.