A health ministry spokesman said the woman died of birdshot wounds. Fellow protesters said she was hit with birdshot fired by police to disperse the march.
Islamists have called for protests tomorrow in a bid to revive what they say was the "revolution" that overthrew Mubarak and briefly brought an Islamist to power as president.
Police have warned they would confront protests "decisively."
Police have cracked down on the Islamists since the military overthrew Mubarak successor Mohamed Morsi in 2013 after a year in power, and hundreds have been killed in clashes.
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Today's central Cairo protest was organised by the Socialist Popular Alliance party.
"The party decided to hold a symbolic protest to commemorate the anniversary of the January 25 revolution," said member Adel el-Meligy.
Police "fired tear gas, birdshot and arrested the party's secretary general and five other young members," he told AFP.