A passenger suffered serious head and facial injuries after a block was thrown through a bus window.
The woman remains in hospital, but her condition is not 'thought to be life-threatening'.
Avon and Somerset Police has now launched an investigation into the attack, which happened at about 8.20pm, on Sunday (October 18).
The bus was travelling on Doncaster Road, in Southmead, and it is thought that the object was thrown from the Southmead Adventure Playground.
A force spokesperson said: "The victim was travelling on the upper deck of the bus when an unknown suspect threw a piece of masonry through the window next to where she was seated.
"She was hit in the face and suffered significant head and facial injuries.
"The victim remains in hospital at this time where she is being treated for her injuries, which are not thought to be life-threatening.
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"Officers are reviewing local CCTV and would like to hear from anyone who witnessed what happened, or has information that could assist their investigation.
"If you can help, call police on 101 quoting reference 5220236602."
