Security forces in Nigeria are on full alert, the government has said, after the vice-president was barred from running in April's presidential poll.
Mr Abubakar has been left off the approved list of election candidates
President Olusegun Obasanjo's office said there were plans to "sponsor protests" and cause violence.
The alert comes as Vice-President Atiku Abubakar is challenging in court the electoral commission's decision to omit him from the list of candidates.
He said his disqualification by the commission was unconstitutional.
President Olusegun Obasanjo's office said the security measures were to "defend democracy" and "ensure a successful transition" to a new administration after the 21 April election.
Mr Abubakar's Action Congress party issued its own statement saying the government should "reinstate all those illegally barred from contesting".
Electoral officials say graft charges against the vice-president would have to be dropped for him to be eligible.
Mr Abubakar, however, told the BBC there was no law barring him from running and that the president was trying to keep him out of the election.
His lawyers argue that as vice-president he has constitutional immunity and should not have been indicted in the first place.
President Obasanjo is standing down after two terms in office
Correspondents say a final decision is unlikely before early April because the parties will fight all the way to the Supreme Court.
If Mr Abubakar wins, the poll may have to be delayed in order for his name to be included on ballot papers which would then have to distributed for Nigeria's more than 60m registered voters.
Mr Abubakar became a leading opposition candidate for the Action Congress after falling out with former ally President Obasanjo.
He refused to support Mr Obasanjo's attempt last year to change the constitution in order to run for a third term in office.
President Obasanjo is standing down after the election and Umaru Yar'Adua is running on behalf of his People's Democratic Party (PDP).
Former military ruler Muhammadu Buhari of the All Nigeria People's Party (ANPP) is the best known challenger on the list of 24 approved candidates.