An "on-going incident" is taking place at Maidstone Prison, said to involve "a number of prisoners". It is said to have been confined to one wing.
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A 61-year-old woman was taken to hospital with head injuries after the incident in Islington, north London.
Counting will begin on Saturday morning, with the result expected later in the day.
There have been a number of incidents involving heavy goods vehicles getting stuck and causing disruption in recent months.