US Secretary of State John Kerry has said he is "deeply disappointed" by Nigeria's decision to postpone next weekend's elections and issued a call for calm.
The six-week delay was announced after security chiefs said the military needed more time to secure areas under the control of Boko Haram.
The Islamist extremists have seized swathes of territory in northeastern Nigeria.
"Political interference with the Independent National Electoral Commission is unacceptable, and it is critical that the government not use security concerns as a pretext for impeding the democratic process," Mr Kerry said in a statement.
The ruling PDP has been pressuring the electoral commission to postpone the polls, while the opposition has been staunchly against a delay.
The postponement to 28 March could now stoke unrest in opposition strongholds such as the commercial capital, Lagos, and Nigeria's second city, Kano.
The poll will pit incumbent Goodluck Jonathan of the People's Democratic Party against former military ruler Muhammadu Buhari of the All Progressives Congress (APC).
It is likely to be the most hotly contested election since the end of military rule in 1999.
'Security cannot be guaranteed'
Independent National Electoral Commission (INEC) chairman Attahiru Jega said: "The commission cannot lightly wave off the advice of the nation's security chiefs.
"The risk of deploying young men and women and calling people to exercise their democratic rights in a situation where their security cannot be guaranteed is a most onerous responsibility.
"Consequently the commission has decided to reschedule the elections thus.
"The national elections, i.e. presidential and national assembly, are to hold on March 28, 2015; governorship and state assembly elections are to hold on April 11, 2015."
Mr Jega said National Security Adviser Sambo Dasuki had written to the INEC last week stating that it could not guarantee security during the original proposed election timetable because of on-going military operations to fight Boko Haram insurgents.
Mr Dasuki called for a delay last month over concerns that not enough biometric ID cards necessary for voting would be distributed in time.
Concerns over security have been raised several times as a reason for a delay.
However, the INEC had outlined red zones where the vote could not be held and alternative polling units for the affected constituencies.
Mr Buhari, who is running for a fourth time against the PDP, believes that he will win.
Mr Jonathan was initially viewed as the likely winner but the momentum has shifted to the opposition in the past few months.
