Nigeria's main opposition parties have called for Saturday's presidential election to be postponed.
President Obasanjo is standing down after two terms
In a statement, 18 parties also threatened to boycott the poll unless several of their conditions were met.
Their demands include the annulment of last weekend's state polls described by the opposition as a "sham", and the disbanding of the electoral commission.
But it is very unlikely the government will fulfil the opposition's demands, says the BBC's Will Ross in Abuja.
Meanwhile, heavy shooting is being reported from the outskirts of the city of Kano, where hundreds of Islamist militants, known as the Taleban, moved in on Tuesday.
The Nigerian army appears to have begun an offensive against the Islamists, who killed 12 policemen and a civilian in an attack on a police station.
Residents are fleeing from the area.
Police said the militants had come from north-eastern Nigeria to avenge the assassination of a radical Islamic preacher who was shot dead at a mosque last Friday.
It is not the first time extremist groups have carried out attacks in Nigeria but is very rare, says the BBC's Alex Last in Kano, and comes as Nigeria is already grappling with instability over the polls.
The opposition parties called for a postponement of Saturday's presidential elections after lengthy talks in the capital, Abuja.
Twelve policemen and a civilian were killed in the Kano attack
In the statement, they demanded the "total voiding" of the 14 April state elections and the complete disbandment and reconstitution of Nigeria's National Electoral Commission.
Nigeria's governing People's Democratic Party (PDP) has won 26 of the 33 states declared so far.
The opposition parties also urged Nigerians to "protest in a non-violent manner these sham elections so far held, and to resist further elections under the current structure".
The opposition said the electoral commission was under the control of outgoing President Olusegun Obasanjo.
The opposition coalition includes Vice President Atiku Abubakar and former military head of state Muhammadu Buhari.
They are regarded as the main challengers to Umaru Yar'Adua - the governing party's presidential candidate.
Apart from attempting to discredit the elections, many observers question what the opposition would achieve by boycotting, our correspondent says.
There has been a worrying spate of violence, including killings, in the run-up to the polls.
Many Nigerians fear further violence ahead of the vote, which should lead to the first handover of power in Africa's most populous nation from one civilian to another.
President Obasanjo is standing down after two terms in office.