UEFA has opened disciplinary proceedings against Porto following allegations of racial abuse during last week's 2-1 defeat at home to Manchester City.
City wrote to UEFA to lodge a complaint after claims of monkey chants directed against Mario Balotelli and Yaya Toure from fans at the Estadio Dragao on Thursday night.
UEFA announced in a statement: "Following a complaint lodged by Manchester City FC, UEFA has today opened disciplinary proceedings against FC Porto for the improper conduct of supporters (article 11 bis of the UEFA disciplinary degulations) during their UEFA Europa League round of 32 first-leg match against Manchester City FC last week.
"The UEFA control and disciplinary body will deal with the case on 29 March."
Roberto Mancini said after the game that he had not heard any abuse during the game, but Balotelli reported the matter to a club official.
On Tuesday morning, Mancini told his pre-match press conference: "I don't know what has happened. I was concentrating on the game and I didn't hear this, but everywhere there are some not clever people - this is life."
When asked if Balotelli had been upset, he said: "I think Mario for this is strong enough. This was the culture sometimes in Italy or France. Mario is strong."