The Alliance of the Revolutionaries of Egypt are calling the event a "million man sit-in."
The Muslim Brotherhood, one of the largest organizations in the nation, has said that it is not having its members join the event.
Besides Cairo, clashes between protesters and police have also reportedly broken out in the cities of Suez and Alexandria.
Military officials have said generally that they will allow protests, so long as they are peaceful.
The Supreme Council, moreover, has stressed its commitment to "handing over power to an elected, civil administration" and said they do not "seek to prolong the transitional period in any way."
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