A smoke bomb was set off inside Kenilworth Road while Luton and Watford players observed a minute's silence in memory of the Duke of Edinburgh on Saturday.
English Football League games were moved from their traditional 3pm kick-off slot due to Prince Philip's funeral taking place at St George's Chapel in Windsor Castle at the same time following his death last week at the age of 99.
As Luton and Watford players stood around the centre circle for a minute's silence before the 12.30pm start, a smoke bomb was let off inside the ground close to the executive boxes and seating.
No players were near to the device when it exploded to reveal red smoke, but an image of it could be seen on a picture taken by Luton's official Twitter account.
Due to the coronavirus pandemic, all EFL matches continue to be played behind closed doors, but it did not stop a slight disruption ahead of the derby fixture in the Sky Bet Championship, with a steward forced to dispose of the smoke bomb before play was able to get under way.
The PA news agency has contacted Luton and the EFL for a comment.