The BBC's Jon Leyne: "They [Masry fans] charged firstly at the players and then the [al-Ahly] supporters"
Clark claimed the scenes were "indicative of something that has been pre-meditated and orchestrated", and "perhaps involved infiltration of the home fans by outsiders".
Clark, 50, is a former Scottish Premier League official who refereed three Scottish Cup finals and three Scottish League Cup finals.
He was invited to take control of a Cairo derby because of his experience of refereeing highly charged matches in Glasgow and Edinburgh.
"The backdrop is that Al-Ahly is perceived as the workers' club, if I can describe them in those terms," added Clark.
"Their supporters have been at the forefront of the protests in Tahrir Square against the old regime and some of the reports coming out of Egypt are suggesting there is something sinister going on in the background.
"The suggestions when I was there were that Al-Ahry were always hard done to because they were seen as representing, effectively, the lower classes.
"But that's something you hear frequently across Europe about different football teams, so I took it all with a pinch of salt at that time.
"Now I'm beginning to believe there is perhaps a bit more credence to it.
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