At least one of those affected by the decision is a witness to the killing of 32-year-old Shaimaa el-Sabbagh, an unarmed protester taking part in a peaceful demonstration.
A police lieutenant faces a manslaughter charge over her killing at the January demonstration on the eve of the anniversary of Egypt's 2011 revolt against autocrat Hosni Mubarak.
Azza Suleiman, an activist who is among those acquitted, said she was at a restaurant across the street when the protest began.
But after Suleiman went to the prosecutor to give a statement about what she saw, the prosecutor charged her in this case - an attempt, she said, to "shut up" people from bearing witness against the police.
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Suleiman said today's verdict was a surprise after just one other session. She's pushing forward with a campaign for a law that protects witnesses and encourages people to testify when they witness government crimes.
Egyptian police have consistently maintained they didn't fire birdshot at the protesters despite overwhelming photographic and physical evidence to the contrary.
Her death also has stoked anger over the perceived brutality of the police and called into question the validity of a law adopted in December 2013 that bans all street protests without prior permits.