Celtic are investigating after clashes between supporters and police during their Europa League match in Stockholm.
Officers were seen using batons, while punches were thrown as fights broke out during the tie with AIK at the Friends Arena.
Celtic said Swedish supporters had entered the away section, from which flares were also lit before kick-off.
A club spokesperson said: "We are aware that a number of Swedish supporters entered the area for visiting fans and we are currently investigating events."
Celtic progressed to the Europa League group stages with a 4-1 win.