Nigeria's election commission announced Tuesday that the opposition party's candidate Muhammadu Buhari won against President Goodluck Jonathan in the closest presidential race in the country's history.

Buhari's party, the All Progressives Congress, has said incumbent president Goodluck Jonathan called Buhari to concede defeat.

Lai Mohammed, the spokesman for the APC said, "President Jonathan called General Muhammadu Buhari, the winner of the elections, to congratulate him … There had always been this fear that he might not want to concede, but he will remain a hero for this move. The tension will go down dramatically … Anyone who tries to foment trouble on the account that they have lost the election will be doing so purely on his own."

Earlier Tuesday, the APC announced that their candidate Muhammadu Buhari has defeated incumbent President Goodluck Jonathan in Nigeria’s presidential election.

However, the election commission had not declared a winner yet. In order to avoid a runoff election, a candidate must not only win a clear majority of votes, but also must win at least 25 percent of the vote in two-thirds of the states.

This was the first election in the history of Nigeria in which there was a strong opposition. The rulling People's Democratic Party (PDP) has been in power since the end of military rule in 1999.

Read more: Nigeria Beyond the Elections

The PDP accused the head of the Independent National Election Commission (INEC) of being biased against the PDP on Tuesday. Godsdey Orubebe, the PDP's collation agent at the national collation center said to the chairman of the national commission, "You are tribalistic. You are selective … We don't believe in you anymore. You are compromised. And we won't take it from you."

Orubebe said that his party has protested the election results in four northern states. The northern states are predominatly Muslim, poorer, and more under the threat from the violent Islamist movement, Boko Haram. Buhari is from the north and the INEC announced that he had won by large margins in those states, from 69 to 93 percent of the vote. The INEC said it had not received any complaints about irregularities in the northern states.

The INEC has begun an investigation into claims by the opposition party, APC, of electoral fraud in Rivers State.

Buhari has been leading against Jonathan in 18 out of the 19 states where the INEC has officially announced results. The only state that Jonathan has won so far has been Rivers State.

The election commission chief said that while there were a few technical glitches, he was sure that they would meet their objective of holding a "free, fair, credible and peaceful" election.