Prisoners stage second protest over conditions at HMP Glenochil
Prisoners are staging the second protest in a week over conditions at HMP Glenochil in Clackmannanshire.
The BBC learned that inmates began demonstrating earlier on Saturday, having staged the first protest last Sunday.
It is understood they are unhappy about the length of time they are allowed to spend outside their cells and are calling for more activities.
The Scottish Prison Service said the protest was being managed safely.
A spokesman for the prison service said: "I can confirm a protest is ongoing at HMP Glenochil, which is currently being managed safely by staff, who will work with those in our care to ensure the security and wellbeing of all."
About 500 inmates serving short sentences have been freed in recent months under an early release scheme, but the prison population is once again heading towards record levels.
Last month the Scottish government dropped plans for the automatic early release of long-term prisoners after concerns from victims of crime.
Instead Justice Secretary Angela Constance said she would bring forward legislation to lower the automatic release point for hundreds of short-term prisoners from 50% of their sentence to 40%.
BBC Scotland News has approached the Scottish government for comment.