Staff have been evacuated from a Melbourne prison where masked inmates carrying makeshift weapons are rioting.
The Ravenhall prison riot started just before 12.30pm and staff were evacuated as a precaution, police said.
"Inmates are contained in the grounds," a police spokeswoman told AAP on Tuesday.
Channel 7's helicopter filmed prisoners carrying sticks, covering their faces and bashing down doors one day before a statewide prison smoking ban comes into force.
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At least six fire trucks are at the prison, on the outskirts of Melbourne's west, after a fire alarm was triggered.
Prisoners are reportedly lighting fires as they roam the grounds.
"Corrections Victoria is investigating a disturbance at the Metropolitan Remand Centre but no further details are available at this stage," a statement said on Tuesday.