UEFA will wait for a report from their delegate before deciding whether to investigate how Red Star Belgrade fans got into their Champions League game at Tottenham.
The governing body banned Red Star's fans from attending Tuesday night's Group B encounter after being found guilty of racism in a qualifying game in Finland in July.
But around 200 supporters made it into the ground and congregated in the North Stand, making themselves heard with chanting before being segregated from home fans by stewards.
Spurs released a statement which 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."
Speaking in the pre-match press conference on Monday, Red Star coach Vladan Milojevic said he was upset his side would not have any supporters inside the stadium.
Milojevic insisted he condemns racism, but expressed a desire to see some away fans in attendance.
"This is the 21st century, it is distasteful to talk about this issue," he said. "I absolutely condemn any racist behaviour.
"On the other hand, I would love to see our supporters here tomorrow. They really do help us, we feel better when they are around.
"I am a man of sport and I simply don't really talk about this, but if you are asking me if I condemn this sort of behaviour, I absolutely do."
SSN's Gary Cotterill at Tottenham Hotspur Stadium
"The Red Star fans are loud, colourful and well behaved. Their presence seemed to take Spurs by surprise - as, to begin with, there was no sign of stewards to segregate them from Tottenham fans, but this eventually happened. There are also 100 or so 'corporate' Red Star supporters, who were officially allowed in. But only in the corporate areas."