Twenty prisoners were killed in Madagascar as the authorities attempted to stop a riot in a prison in the south of the island, the justice ministry said.

The trouble began at the prison in Farafagana at around 12:00 local time (09:00 GMT) with inmates splitting into two groups and attacking guards with stones, the ministry outlined in a statement on Facebook.

They also seized a gun.

Along with the 20 who were shot dead, eight prisoners were seriously injured as the authorities tried to regain control.

Twenty-nine others were also recaptured but by 16:00 local time 31 inmates were still free, according to the justice ministry.

It is not clear how the violence began but in 2018 Amnesty International reported that Madagascar's prisons were "dilapidated, ill-equipped, with [a] lack of financial, material and general support".

It also highlighted how people can spend years in pre-trial detention.