Firefighters joined their colleagues from the police at HMP Ranby where a disturbance involving prisoners is ongoing.
Shortly after 7.30pm around 15 firefighters arrived outside the prison and began putting on their full kit before walking in through the main gate.
Officers carrying shields and other equipment were also seen going into the prison earlier.
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Cloud and outbreaks of rain will ease away eastwards during the morning, clearing the east coast around midday with sunny spells developing.
Police and emergency services are attending the scene.
Rebel-held east remains at risk while the pumping station supplying the western part of the city is once again in operation.