UEFA has opened an investigation after Inter Milan supporters were denied entry to Tuesday's Champions League tie with Porto despite having tickets for the match.
Footage emerged on social media ahead of the last-16 second leg showing hundreds of fans queueing to get into Estadio do Dragao as the game kicked off.
Porto had announced earlier in the day that Inter spectators who had purchased tickets in the home end, even if through the club, would be turned away.
But Inter claim an agreement was reached for their fans to attend the game if they wore neutral colours.
European football governing body UEFA confirmed on Wednesday that the matter is being looked into.
“UEFA had been made aware that a large contingent of visiting supporters had procured tickets in home sectors of the stadium," a spokesperson said.
"The UEFA regulations stipulate that five per cent of the stadium capacity must be provided to the visiting team in a segregated area for their supporters.
"The responsibility for the safe and secure management of spectators and the associated ticketing policy are determined by the match organiser and the relevant authorities.
"Mitigation measures were discussed between both clubs. UEFA is currently looking into the matter."
Inter claimed a goalless draw to advance through to the Champions League quarter-finals as 1-0 aggregate winners.
The Italian club's chief executive Beppe Marotta dedicated the result to those who were unable to get into the ground.
"We celebrate this passage that Inter had not achieved for 12 years, we give the credit to our coach, our players and the fans who were present," he told Sky Italia.
"I also want to spare a thought for circa a thousand fans who were left outside the stadium even after regularly purchasing a ticket.
"I saw footage of families who were locked outside, children crying who had flown here from Italy.
"Football must represent unity and joy, so this is a bitter page in what was otherwise a wonderful evening.
"We had met with the local authorities this morning and they assured us the fans would be allowed in even beyond those limits of the away section. Instead, this did not happen.
"These were for the most part families with children, who certainly weren't here to perpetrate acts of violence, but to support their team.
"I don't believe the situation was so serious as to ban their entry.
"We realise UEFA are not to blame, but we feel that with a formal complaint we can prevent this sort of thing ever happening again."
