Olympiakos fans have been accused of throwing objects on to the pitch and setting off fireworks during the first leg of their Europa League play-off against Burnley in Greece.
Disciplinary proceedings have been opened against the Athens side by Uefa, who also said stairways were blocked.
Olympiakos won Thursday’s match 3-1, with the return leg taking place in Burnley on 30 August.
The disciplinary case will be dealt with by Uefa on 27 September.
A Burnley fan was stabbed and four others were injured in incidents in Athens before the match.
After the game Burnley manager Sean Dyche accused match officials of succumbing to pressure from Olympiakos players.