UPDATE 6-Nigeria's Buhari wins second term as president -electoral commission results

Nigeria's President Muhammadu Buhari on Tuesday won a second term at the helm of Africa's largest economy and top oil producer, a tally by Reuters based on electoral commission results showed.

Buhari had a commanding lead of 56 percent compared to 40 percent for Atiku, before the final district was announced. A message posted on Buhari's Twitter feed late on Tuesday showed him smiling and surrounded by applauding staff at his campaign office.

"I met the very hardworking members of our team, many of them young people, and was briefed on the performance of our party so far in the Presidential Elections. I am very proud of what has been accomplished," he said on Twitter. Earlier, the party of Buhari's main rival, businessman and former vice president Atiku Abubakar, demanded an immediate halt to the release of results by the Independent National Electoral Commission until turnout figures are provided to the competing parties.

Atiku's party has rejected the tallies announced so far as "incorrect and unacceptable". Buhari's party has said the opposition was trying to discredit the returns from Saturday's election. The accusations have ratcheted up tensions in a vote marred by delays, logistical glitches and outbreaks of violence.

Observers from the Economic Community of West African States, the African Union and the United Nations appealed to all parties to await the official results, expected later this week, before filing complaints. The candidate with the most votes nationwide is declared winner as long as they have at least one-quarter of the vote in two-thirds of Nigeria's 36 states and the capital, Abuja. Otherwise there is a second-round run-off.

Buhari has already secured enough votes to meet both requirements, the Reuters tally showed. (Reporting by Paul Carsten, Alexis Akwagyiram, James Macharia, Camillus Eboh and Felix Onuah; writing by Alexis Akwagyiram and James Macharia; editing by Grant McCool)