Nigeria's opposition parties have asked the parliament to annul last weekend's presidential poll and set up an interim administration to take over government.
Gen Muhammadu Buhari won 18% of Saturday's vote
They also called for a re-run of all recent elections described as "flawed" by local and international observers.
Although leading opposition candidate Muhammadu Buhari attended the meeting, Vice-President Atiku Abubakar was represented by his running mate.
Ruling party candidate Umaru Yar'Adua won by a landslide in the polls.
The BBC's Senan Murray in the capital, Abuja, says that having failed to present a common front during the poll, it is not clear how the opposition can now challenge its outcome in a unified way.
Earlier, the powerful Catholic Bishops Conference of Nigeria advised against any mass protests, saying it is best to head for the law courts.
At the meeting, opposition parties agreed to have special prayers in mosques and churches over the weekend for some 200 Nigerians who died in election-related violence.
They also called on Nigerians to troop out to the streets on 1 May to reject the "sham elections" and call for their cancellation.
Fearing a possible outbreak of violence in the volatile Kaduna State, a ban on street demonstrations has just been announced in the north-western state.
But further north in the conservative Muslim-dominated Kano State, some women heeded opposition calls and took to the streets to protest the outcome of last Saturday's parliamentary poll.
The Catholic bishops admitted Nigerians' votes had been "abused, traumatised and brutalised", but also said the answer did not lie in violent protests.
"Two evils never make a right. To cause chaos; to cause people to lose their lives and property is definitely wrong," Archbishop Alaba Job told the BBC.
Mr Yar'Adua gained 24.6m votes, against 6.6m for his closest challenger, Mr Buhari and 2.6m for vice-president turned opposition candidate Mr Abubakar.
In addition to his election troubles, Nigeria's Code of Conduct Tribunal is expected to decide whether it could try Mr Abubakar for graft despite his constitutional immunity against criminal prosecution.
Outgoing President Olusegun Obasanjo has defended the organisation of the vote.
"No elections in the world will ever be regarded as perfect... You cannot use European standards to judge the situation in a developing country," he told the BBC.
The presidential poll was held alongside elections for the National Assembly and Senate.
Nigeria - one of the world's biggest oil producers - is of key strategic interest to both the West and the growing economies of the East.
But despite the country's huge oil wealth, much of the population lives on less than $1 a day.