Liverpool fans were subjected to "heavy-handed" treatment by police and stewards in Seville on Tuesday, supporters' group Spirit of Shankly (SOS) has said, with the Premier League club launching its own investigation into the claims and UEFA confirming it is aware of the situation.
SOS alleged that some away fans were refused entry to the Champions League match and were assaulted by police and stewards at the Ramon Sanchez Pizjuan Stadium.
"We became aware of issues affecting supporters during the first half of the match," SOS chair Jay McKenna said in a statement to ESPN FC.
"A couple of our committee members reported problems getting into the ground, being refused access as the flag they had [which featured a Liver bird and the word "defiance"] was accused of having a political message.
"What is more concerning is the reports of heavy-handed treatment of supporters, including physical assaults, with one committee member reporting she was punched as she was pushed out of the ground.
"Similar experiences have been shared with us from other supporters last night, and we've asked people to come forward so we can raise this issue with the club.
"We've already flagged it up to them and we're aware they are looking for more information.
"Clearly, though, what happened last night isn't acceptable and we will be asking both clubs and UEFA what they plan to do about it."
UEFA, European football's governing body and the organiser of the Champions League, told ESPN FC: "We are aware that there were some issues at the away supporters' entrances in regards to fans accessing the stadium" and said it had no further comment.
Liverpool released a statement later on Wednesday which read: "Following detailed and troubling accounts given by Liverpool supporters attending the match against Sevilla last night, the club is seeking to establish the facts regarding their treatment at the hands of the host stewards and local police force.
"The safety and security of our supporters is our paramount concern and we intend to gather all the relevant information before responding further."
A police spokesman told ESPN FC: "Having spoken to Sevilla FC security coordinator (a police chief), there were no incidents or police charges made in the two days prior to the game or during the day of the game.
"The only incident was that some Liverpool fans tried to pass through the turnstiles to access the Sanchez Pizjuan stadium with false tickets, colour photocopies of tickets and when they tried to scan the bardode it wouldn't work. Sevilla's private security had to get them to move aside so that other fans could enter.
"With respect to the flags, norms request that banners and flags that are more than 3x2metres in size, must have an accredited sticker and that the pole is flexible.
"The police received a file of the official Liverpool flags that complied with the requirements and if one did not, then it wouldn't have been allowed to be taken in.
"Moreover, if a flag or banner has a message that incites violence, then it would not be allowed in but not just yesterday, in any other game whether in La Liga or European competition."
Article 6, section b, of Spain's law against violence in sports, which was updated earlier this year, states that it's prohibited to "enter, show or make banners, flags symbols and other signs with message that incite violence."
Article 6, section c, of Spain's law against violence in sports states: "it is prohibited to enter the sporting ground under the influence of alcohol, toxic drugs..."
ESPN FC's Spain correspondent Adriana Garcia contributed to this report