Armed members of a Nigerian Islamist group known as the Taleban have stormed a police station in the northern city of Kano, killing at least 13 people.
The Taleban attacked police stations in 2004
A police source told the BBC there were between 300 and 500 people, some of whom were heavily armed.
The army has been sent in and police have evacuated the area.
The violence comes ahead of Saturday's presidential elections and follows the shooting dead of prominent Islamic cleric Ustaz Ja'afar Adam last Friday.
A curfew was imposed in Kano following the elections for State Governor on Saturday.
Mr Adam had been a member of the Kano State government but fell out with the governor.
The city was tense after the weekend's state governor elections but the situation had reportedly calmed after the opposition candidate was declared the winner.
Nigeria's Taleban sect attacked police stations in the remote north-east three years ago but has since largely disappeared from public view.
Earlier, the Independent National Election Commission (Inec) announced that Vice-President Atiku Abubakar would be on the ballot paper for Saturday's poll, incompliance with Monday's Supreme Court ruling.
However, Inec have not said how this will be done at such short notice.
The BBC's Alex Last says it is possible that a sticker with Mr Abubakar's name could be added to the ballot papers.
Earlier, police announced they had banned all open-air rallies ahead of the elections, according to state radio.
The move is intended to prevent breaches of security, after at least 21 people died in clashes in last week's elections for state governor.
Mr Abubakar has met the other main opposition candidate Muhammadu Buhari to discuss a possible alliance against the ruling party's Umaru Yar'Adua.
But a BBC correspondent says there is little chance that either man will agree to drop out.
Mr Abubakar was a founder member of the ruling People's Democratic Party (PDP) but fell out with President Olusegun Obasanjo last year when he opposed moves to let the president seek re-election.
The PDP has won 26 of the 33 states declared so far, including most of the Niger Delta.
The presidential contest should see the first time one civilian administration hands power to another since Nigeria's independence from Britain in 1960.
President Obasanjo is standing down after two terms in office.