Nigerian Police said the fans died after a cable fell on them at a viewing centre in the city of Calabar, capital of the Cross River state.
According to reports, a high tension electric cable that was very close to the viewing centre fell on the building.
The number of deaths and injuries are yet to be confirmed, but the News Agency of Nigeria say there has been at least 30 killed, with many more injured.
Manchester United and the British High Commission in Nigeria have both paid tribute to those who died in the accident.
The club tweeted: "Our thoughts go out to the United fans, their friends and families affected by the tragedy in Calabar, Nigeria, yesterday."
The British High Commission in Nigeria added: "Our deepest sympathy for the loss of lives in Calabar during a Europa League football match."
Ben Ayade, governor of Cross River, said: "The incident marks a dark chapter in the life of the state where the precious lives of football lovers have been lost.
"At this point in time, it is sad to know that the victims lost their lives at a viewing centre where they usually converge, at their leisure time to watch their matches."