Lancashire police have launched an investigation into an incident at Saturday's game at Deepdale, Preston, in which a smoke bomb was let off in the area where Coventry City fans were watching the game.
The incident, which occurred in the Bill Shankly Kop, happened in the second half, but no-one was hurt and no damage has been reported.
A spokesman for Preston North End FC said: "We can confirm that during the second half, a Coventry fan at the top of the stand was responsible for discharging a smoke bomb which emitted a small quantity of blue smoke."
"Nobody was detained at the time but Preston North End security officers and Lancashire Constabulary are investigating the matter."