UEFA has opened disciplinary proceedings against Arsenal, Galatasaray, Basel and Borussia Dortmund following crowd disturbances during Wednesday night's Champions League ties.
Several flares were thrown onto the pitch from the section where the 3,000-strong travelling Galatasaray supporters were located towards the end of the first half at Emirates Stadium, causing the Group D match against Arsenal to be briefly halted.
Metropolitan Police confirmed there had been six arrests as part of their operation for the European tie in North London, two of which were for entering the ground with flares.
UEFA view it as the home side's responsibility to search supporters for any illegal items before they come into the stadium.
At St Jakob Park, a battery appeared to be thrown in the direction of the assistant referee near to the Basel goal during the tie with Liverpool, while at Anderlecht's stadium, flares were lit in the area housing travelling Borussia Dortmund fans.
If found guilty, the clubs could be hit by a fine from the European governing body, or be ordered to play matches behind close doors.
An updated statement on UEFA's website, issued on Thursday evening to add reference to Arsenal's charge for what is understood to be a minor possible sanction, read: "Disciplinary proceedings have been opened following incidents at last night's UEFA Champions League matches:
"Match: UEFA Champions League Group B match between Basle and Liverpool (1-0) played on Wednesday 1 October in Switzerland.
"Basle: Throwing of missiles (art. 16 (2) (b) DR)
"Match: UEFA Champions League Group D match between Arsenal and Galatasaray (4-1) played on Wednesday 1 October in England.
"Galatasaray: Crowd disturbances (art. 16 (2) (f) Disciplinary regulation); setting-off and throwing of fireworks (art. 16 (2) (b) & (c) DR)
"Arsenal: Insufficient organisation (art. 16 (1) DR)
"Match: UEFA Champions League Group D match between Anderlecht and Borussia Dortmund (0-3) played on Wednesday 1 October in Belgium
"Borussia Dortmund: Crowd disturbances (art. 16 (2) (f) Disciplinary regulation); setting-off and throwing of fireworks (art. 16 (2) (b) & (c) DR)
All three cases will be dealt by the UEFA Control, Ethics and Disciplinary Body on October 16."