Nigeria's main opposition Peoples Democratic Party (PDP) on Monday accused the government trying to manipulate results of last weekend's presidential election.
Party chairman Uche Secondus said the ruling All Progressives Congress (APC) of President Muhammadu Buhari was colluding with the electoral commission.
The APC was using "impeachments, manipulation, incarceration", state agencies and ministers to rig the outcome, he charged.
The accusation came as the Independent National Electoral Commission (INEC) announced the first results from Saturday's election, at which Mr Buhari, a former military ruler, is seeking a second term.
Analysts have been split about who will win -- the PDP candidate, former vice president Atiku Abubakar, is mounting a strong challenge.
Mr Secondus told a news conference at Mr Abubakar's campaign headquarters that it would be "difficult for the entire country to accept this level of intimidation".
The chair called on INEC to "live up to its responsibility."
"They are under pressure, and the situation we find ourselves in, it's as if we are in the military era," he said.
"We have never seen this before, where at the end of the election, the (presidential) villa will dispatch security agencies and ministers to all parts of the country to mount pressure on governors," he told reporters.
Mr Secondus said the party believed it was an "attempt by the government and other agencies to manipulate the result".
The PDP and the APC have both accused the other of colluding with INEC to rig the result, after the commission announced a last-minute delay to polling scheduled for February 16.
There was no immediate comment from INEC or the APC.