In another call with Sam, he was more specific about the nature of the violence in Tahrir Square, which some of his video clips capture.
A peaceful march within Cairo was planned today in reaction to the Egyptian government's prior refusal to allow the group to travel to Gaza. Police in Cairo seem to have moved to prevent even that.
As others have mentioned through Tweets and other mediums, a protest in Tahrir Square, near the Cairo Museum and hotels where some participants are staying, and from where Sam has been posting, has turned violent.
Sam says "they brought in the henchmen," "not the regular police" and began to "pull people back" and people have been beaten. There are reports on Twitter of demonstrators being bloodied, cameras being smashed.
[Added: This was featured in an IPA news release titiled "Americans Attacked in Cairo over Gaza March; Video Available."]