Tottenham Hotspur said they would take the strongest possible action after several hundred Red Star Belgrade fans defied a UEFA ban to attend the Champions League Group B tie on Tuesday.
Red Star fans were barred from buying tickets for the clash in north London as punishment for racism by some of their fans during a Champions League qualifier against HJK Helsinki.
Yet a boisterous contingent were making themselves heard in the upper-tier behind the goal, although they did not have much to cheer as their team were hammered 5-0.
It is not clear how they obtained the tickets but Tottenham said they would come down hard on any home fans who sold their ticket on to visiting fans.
A Tottenham spokesman said: "Red Star supporters illegally obtained individual tickets on Level 5 and made their way around to one section of the stadium in order to congregate as a group.
"Stewards and police contained the group as it was too large a number to eject. We have launched a full investigation in order to understand how tickets have been acquired.
"We shall take the strongest possible action against any individual found to have sold their ticket on."
A UEFA spokesman said they would await on reports before deciding on any action.