The comments drew the backing of his ruling All Progressives Congress (APC) party
Nigeria's main opposition candidate accused President Muhammadu Buhari Tuesday of paying "lip service" to democracy, as a war of words escalated before rescheduled elections this weekend.
The country's election commission called off the vote just hours before it was due to start last Saturday, blaming logistical difficulties in the distribution of materials.
Since then, the tone of the two main parties has become increasingly combative, despite both sides committing to hold peaceful elections and to accept the results.
Buhari, 76, a former military ruler who came to power as civilian president in 2015, said Monday he had ordered the security services to be "ruthless" with vote-riggers.
"I am going to warn anybody who thinks he has enough influence in his locality to lead a body of thugs or snatch (ballot) boxes or to disturb the voting system, he will do it at the expense of his life," the president said.
The comments drew the backing of his ruling All Progressives Congress (APC) party, with violent ballot-snatching a feature of elections in Africa's most populous nation.
But Atiku Abubakar, a former vice-president from the Peoples Democratic Party (PDP), said Buhari's words were "a painful reminder of the era of dictatorship and military rule".
"I have always said that General Buhari is not a democrat. He's never been a democrat... It's lip service. He's more of a power monger than a democrat," he told senior PDP members.
He added: "It's quite shocking for any head of state to utter such words in a democracy and we will not accept it from General Buhari in this country."
Abubakar, 72, said a "pre-condition for free, fair, and credible elections is that the people are able to cast their votes without fear or violence".
The 2015 election, which saw Buhari become the first opposition candidate in Nigerian to defeat a sitting president, were also considered the country's most credible.
Abubakar told Buhari "you owe it to our people to let them give their verdict in the same way you were elected".
"If you do so, history will commend you for it. If you do not, history will condemn you for it. But before history does it, we will condemn you for it."
Some 84 million Nigerians are registered to vote on Saturday.