Nine-time Russian champions Spartak Moscow on Thursday accused fans of their Champions League play-off rivals Fenerbahce of hacking into the club's website and threatening violence.
The charges stemmed from a fiery first leg in Moscow on Tuesday, which the Russians won 2-1 after their fans set fire to a portrait of Kemal Ataturk, the first president of modern Turkey.
"The Father of All Turks" has the effective status of Turkey's national symbol, and portraits and pictures of him on horseback adorn family rooms and buildings across the country.
The Russian government's official newspaper said the Fenerbahce fans had hacked the website overnight on Wednesday and posted a message in Turkish reading: "We do not let crime go unpunished."
Spartak are due to travel to Istanbul for the second leg next Wednesday.
The Spartak site was running again by Thursday afternoon after the club used its Twitter account to blame the incident on "Turkish hackers".
The winner of the head-to-head encounter will qualify for the group stage of Europe's elite club competition.