ABUJA, Nigeria (AP) " Nigeria's president and his top challenger in Saturday's election have renewed a pledge of peace ahead of a vote marked by heated accusations that have alarmed some observers.

President Muhammadu Buhari, who seeks a second term, and former Vice President Atiku Abubakar on Wednesday vowed to contribute to a free and fair election in Africa's most populous country and refrain from "religious incitement" or ethnic profiling.

Past elections in Nigeria have been marked by deadly violence, though the previous vote in 2015 was relatively calm. It saw the first defeat of an incumbent and first democratic transfer of power between parties.

In this campaign the top two parties have hurled accusations of vote-rigging plots.

With just a few days left before the vote, troubles have multiplied. Nigeria's election commission reported that one of its offices had burned with more than 4,600 smart card readers meant to be used in the voting "destroyed in the inferno." This is the third such fire in the past two weeks.

Four people were crushed to death in a stampede on Tuesday during a ruling party rally in Rivers state in the south, seen as one of the potential hotspots of election violence, according to Nigerian media reports.

The push for candidates to publicly renew their peace pledge, first made in December, picked up in earnest in recent days after the governor of Kaduna state declared on television that anyone who came to Nigeria to intervene in the election "would go back in body bags."

Election observers from the European Union quickly protested the comments and clarified their role as impartial. A number of foreign observer teams are set to monitor the vote. With tempers high, local media report that Nigeria's inspector general of police has assured protection to foreign election observers.

Another spat erupted following a statement by the U.S. ambassador in which he warned that anyone inciting violence would be held to account.

The ruling party objected to the ambassador's remarks, issuing a statement saying that he "seeks to shrink our ambit of free speech so that we may behave in accordance with their vision of well-behaved Africans."

In recent days, Nigeria's security forces have pledged to do their best for a peaceful election. Police this week said they would detach officers accompanying top officials to free them up for wider duty and to avoid any "undue advantage."

At stake in Nigeria's election is a vibrant country of some 190 million people, including more than 84 million registered voters, that is Africa's largest economy and oil producer. And yet the sluggish economy is still recovering from recession and excessive dependence on crude oil exports, while high unemployment, corruption and poor infrastructure continue to hamper Nigeria's vast potential.