They were electrocuted by a high-tension cable that fell inside the viewing centre during the game.
As the usual in Nigeria, fans gather to watch the second leg quarter-final clash on Thursday.
According to several reports, a high-tension cable fell inside the viewing centre at the Iyang-Esu of the Calabar Municipal Local Government causing an explosion which killed about 30 and injured many.
There were about 80 fans watching the game at the business centre according to reports.
“It was a horrible sight to behold. I wish I didn’t come out to watch the match. Come to think of it, I have DStv at home but I enjoy watching matches at viewing centres,” a football fan who was in the viewing centre when the incident happened told Punch.
“I could have been dead. I can’t believe that the people I was chatting and joking with a few minutes ago are all gone in a most anguishing way. This world is vain.”
A few injured survivors were taken to the hospital.
The Cross River State Police Command also confirmed the incident and said the exact number of casualties would be confirmed later.
Manchester United beat Anderlecht in the game courtesy of goals from Henrikh Mkhitaryan and Marcus Rashford to progress to the Europa League semifinals.