Egyptians are protesting the interim military government wanting them to step aside immediately. That would leave a power vacuum and the result could be chaos. I'm skeptical that this isn't deliberate, and if it isn't it's certainly playing perfectly into some party's hands. It isn't necessarily the Muslim Brotherhood but it certainly could be.
What I saw was a stage, a massive banner backdrop that clearly was professionally done, and other placards that also look professionally done. What's behind that? Money. And behind money, is organization.
The possible outcomes from the protest include this (via HuffPo of all places);
The military has rejected calls to immediately step down, saying its claim to power is supported by the warm welcome given to troops who took over the streets from the discredited police early in the anti-Mubarak uprising as well as an overwhelming endorsement for constitutional amendments they proposed in a March referendum.
Tantawi has offered another referendum on whether his military council should step down immediately.
Such a vote, activists say, would be divisive and likely open the door for a deal between the military and political groups, particularly the Muslim Brotherhood. Egypt's largest and best organized group, the Brotherhood is notorious for its opportunism and thirst for power. It was empowered after the fall of Mubarak, regaining legitimacy after spending nearly 60 years as an outlawed group.