A surprise delay in Nigeria's presidential ballot has served foreign asset managers another reminder of political risk and volatile investment returns in Africa's biggest economy - just days after piling into assets in a bet on a smooth election run.

The electoral commission announced a week's delay to voting in the early hours of Saturday, citing logistical problems, even as some of Nigeria's 84 million registered voters were already making their way to polling stations.

The vote pitches President Muhammadu Buhari against former vice president Atiku Abubakar, in what is seen as a tight race.

The delay adds to uncertainty for investors, who have endured a wild ride in the West African country: The 2014 oil price crash, and election in 2015 followed by currency controls and dollar shortages that tipped the oil-exporting economy into recession in the same year, its first in more than two decades. Its bonds got ejected from key indexes.

A new exchange rate mechanism launched in 2017 drew back some investors but concern has built around the election that has proved hard to call, threatens to spark violence and promises little material change.

"The likelihood of violence is now higher than before," said Thierry Larose at Vontobel Asset Management. "And we have seen some effect on markets."

•A cyclist drives pasts a campaign poster for President Muhammadu Buhari in a street after the postponement of the presidential election in Kano, Nigeria February 17, 2019. REUTERS/Luc Gnago/File Photo