Thousands of Celtic supporters flew Palestine flags ahead of a match despite being asked not to do so by the club.
A display in the colours of the flag was in the lower half of the standing section, with thousands of smaller flags seen throughout the ground on Wednesday evening.
Two dozen large flags were also flown before Celtic and Atletico Madrid players ran on to the pitch for their Champions League clash.
It’s likely that Celtic will now face disciplinary action from UEFA, and the club was previously fined £9,000 in 2016 after fans flew Palestinian flags during their 5-2 victory against Israel’s Hapoel Be’er Sheva.
A group called the Green Brigade had encouraged Celtic fans to “courageously fly the flag for Palestine” as the Middle East conflict continues to escalate.
But before the match, the Scottish side had urged supporters not to display “banners, flags and symbols” relating to the conflict.
In a statement, Celtic said: “We have witnessed death, violence and destruction in the Holy Land in recent weeks, with thousands of people – men, women and children – killed, injured and displaced.”
The club said they were praying for peace and humanitarian support in the region, and many of their staff and supporters had been affected by recent events.
“Against this backdrop of conflict and pain, sport can promote peace and demonstrate humanity and empathy for all who continue to suffer,” the statement added.
Celtic players and coaches had worn black armbands as a “show of respect and support for all those affected by the conflict” – with the club donating to the International Committee of the Red Cross.
And while the statement said everyone is entitled to their personal views, Celtic stressed that it wanted to be “open to all”.
The Green Brigade, which has been suspended from getting away tickets following recent incidents, said it had distributed the flags outside the stadium.
“We respect the right of all fans who wish not to participate in such an action, however equally we ask that the same respect and freedom is afforded to all fans who do,” the group said.
The Green Brigade raised £130,000 in an online campaign after the 2016 fine, with the money going to medical aid for Palestinians and the creation of a football academy in Bethlehem under Celtic’s name.
Last night’s match against Atletico Madrid ended 2-2, meaning it has been more than six years since Celtic won a Champions League fixture.
Manager Brendan Rodgers refused to be drawn on the pro-Palestine display at a news conference following the game.
“I am only thinking about the football,” he told reporters.