Diouf, a hate figure for opposition fans wherever he goes, refused to hide his feelings after Blackburn's creditable draw, stripping down to his shorts and waving with delight at City fans as he walked off.
His behaviour earned a chorus of boos from City fans and a bottle was hurled at him and picked up by team-mate Pascal Chimbonda.
Greater Manchester Police have launched an investigation. No player was hit and there were no reports of any injuries. The bottle has been seized by police and an investigation is under way.
Controversial figure: Blackburn Rover's El Hadji Diouf goes in hard on City's Adam Johnson before goading fans at the final whistle
The pair then continued to celebrate all the way to the tunnel, lifting their arms up in triumph towards the City fans, with Senegal international Diouf blowing kisses until a policeman came over and hurried him along.
Diouf, who was involved in a row with City's James Milner over a diving incident late in the game, has been a controversial figure since being signed by Liverpool in 2002.
He was fined £5,000 for spitting at a Celtic fan during a UEFA Cup game against Liverpool but went on to rescue his career under Sam Allardyce at Bolton and Blackburn.