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.
Sam's glasses were broken or lost in the turmoil.
Sam guesses about a 1,000 people or more are participating in the demonstration.