Both vigils will begin at 6 pm.
The Derry vigil will take place at the Guildhall while the Belfast event taking place at the gates of the City Hall.
The 16-year-old schoolgirl was found lying wounded on a path in Linear Park in Culcheth, Cheshire, last Saturday.
This morning, police confirmed that two 15-year-olds, a boy and a girl, have been charged with the murder of trans person Brianna.
They are expected to appear in court later for a first remand hearing.
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