Police Scotland is investigating the "significant damage" caused to seats and it counted 18 smoke bombs, three fireworks and one flare being set off. No arrests were made inside Fir Park.
Celtic have already begun their own inquiries into the "appalling" events, and the Scottish Professional Football League has indicated it could take action against Celtic.
The middle lower area of the South Stand at Fir Park, just above where a Green Brigade banner was displayed, was left needing repairs estimated to cost £10,000.
A flare was let off in the same area before the game while two green smoke bombs were thrown into the goalmouth, one of them landing yards from Celtic goalkeeper Fraser Forster.
A missile also flew from the Celtic support not far over the head of the England goalkeeper just after Kris Commons opened the scoring in Celtic's 5-0 Scottish Premiership triumph.
Now the Green Brigade section of the Celtic support, has described the behaviour at Fir Park as "indefensible" and made it clear it does not condone Friday's trouble.
"We as a group are as disappointed as any about such indefensible behaviour and regret that it happened on our watch.
"Whilst it was not members of our group behaving in this manner, we accept the fact that these actions took place behind our banner in what was an unofficial 'Green Brigade section' and that the fans within this area are associated to our group whether they are members or not," it said.
"It is particularly disappointing given how hard we have worked as a member organisation of Fans Against Criminalisation and of our own accord to fight the criminalisation of football supporters as we realise the damage that the events of Friday night could have on our efforts to challenge the continued harassment of Celtic fans by Police Scotland.
"Ultimately, we should have had greater control of the bodies present within our block and our failure to do so has resulted in events which are unacceptable and we will strive to ensure that such scenes are not repeated."
Last week Celtic boss Neil Lennon condemned banners that were displayed at Celtic's Champions League defeat by AC Milan with a slogan depicting Scottish warrior William Wallace and IRA hunger striker Bobby Sands.
The Green Brigade, which was responsible for the display, defended its right to "cultural and political expression".
A spokesman for the main Celtic fans' group, the Affiliation of Registered Celtic Supporters Clubs, said: "We should be discussing another devastating Celtic performance on the pitch instead we are talking about disappointments off the field.
"Let's make no mistake that incidents like Friday night at Fir Park and the publicity that this attracts makes any negotiations regarding ticket pricing, safe standing and policing at games more difficult.
"Any payment for damage and fines that are the responsibility of the club are ultimately paid by us. This is a measurable cost. The damage to our reputation as a support is unquantifiable.
"In a time when we are fighting to reduce ticket prices, fighting against the Offensive Behaviour Act and trying to make the game more attractive to the thousands who fans who have lost interest (Friday) night is disheartening and does nothing to help."
Celtic described the Fir Park scenes as "appalling".