A bomb blast has killed at least six people at the office of Nigeria's election commission in the central town of Suleja, hours before polls are due to open for parliamentary elections.
A BBC reporter in Abuja says it is likely many people were in the office, preparing for voting on Saturday.
Officials said the blast appeared to be a bomb which struck a day after one person died in a bomb blast in Kaduna.
Gunmen meanwhile killed four people in the northern state of Borno.
The dead included an official from the ruling People's Democratic Party (PDP), who had been preparing to distribute election materials on Friday hours ahead of the parliamentary polls, police say.
Several other explosive devices have been defused by the security forces, who had earlier warned of a plot to disrupt the election.
Voting was supposed to begin last weekend but was delayed in some areas.
The blast hit Suleja - just 20 km (12 miles) from the capital Abuja - at about 1800 (1800 GMT), and a senior election commission official told the BBC he was making frantic efforts to contact election workers who had been working in the building and were still unaccounted for.
Nigerian President Goodluck Jonathan condemned the "heinous bomb attack" and ordered an immediate increase in security at all electoral commission premises across the country.
A statement from the president's office said the dead included members of the National Youth Service Corps who had been "engaged in preparatory work for the conduct of free, fair and credible elections in the country".
The BBC's Caroline Duffield in Nigeria says Suleja was the target of a bomb some weeks ago, when men hurled a bomb towards an election rally from a moving car.
The build-up to Nigeria's elections has been violent, adds our correspondent, with attacks on party offices in the Niger Delta, bomb blasts, and the assassination of an election candidate in the north-eastern city of Maiduguri.
At least 85 people have lost their lives in political violence linked to the elections, according to Human Rights Watch, the campaign group.
Voting had already begun last Saturday, and millions were queuing, when it was discovered that ballot papers were missing in some parts of the country.
The vote was postponed again in those areas, because it was not possible to replace ballot papers in time. The elections for president and state governors have also been set back.
Previous elections held since the 1999 end of military rule have been characterised by allegations of widespread fraud and violence.
Presidential elections have been put back a week to 16 April, with polls to choose the 36 powerful state governors now to be held on 26 April.
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.