Manager Brendan Rodgers is calling for the Green Brigade to leave politics out of Celtic Park.
Uefa charged Celtic over an "illicit banner" displayed by the fans' group during their Champions League tie against Linfield,
And the Green Brigade section of the stadium will be closed for the next two games "amid serious safety concerns".
"Celtic is not a political arena for any supporters to display any sort of political element," said Rodgers.
"My opinion is totally aligned with the board and the people running the club. I was saddened by what I saw."
The Green Brigade area will be closed for Wednesday's Champions League qualifier against Rosenborg and against Hearts on the opening day of the Premiership season on 5 August.
However, Rodgers was keen to stress the value of the group's support.
"I want to defend the guys at some time," he added.
"I have been fortunate enough to be around some of the top football clubs in Britain and I can categorically say the atmosphere inside Celtic Park, not just by that section, by the 60,000 supporters is the best you will witness.
"The guys in the corner add an energy and youthfulness which is absolutely amazing.
"The club have worked tirelessly over the years, the first club in Britain to put in a standing section, and 99.9% of the time they, along with the rest of the fans, give us this incredible support.
"But the political element is not acceptable. There are forums, platforms for that outside of football. Take that somewhere else.
"It is not for inside Celtic Park. It is not what the values are, the ethos, and it is certainly not what I'm about as a manager."
The standing section occupied by the Green Brigade was opened at the start of last season and Rodgers stressed the importance of safety.
"Some supporters might say there is a political element to football and a basis to tell a story," he said.
"There might be other countries where this ultras factor goes beyond football.
"But the governance here in Britain is different and one of the big points for me is the safety element.
"If you are putting flares up in a ground, you are open then to things that can happen. You might say that they never happen but they can happen.
"My message is, stick to football, stick to supporting the team that you love."