Avon and Somerset Police were called at just before 4.30am today by the Great Western Ambulance Service to Great Stanhope Street in Bath following a report of a woman falling from a window.
An 18-year-old woman has been taken to hospital with serious injuries and is in a critical but stable condition.
Officers are investigating the circumstances leading up to this incident and as part of their inquiry, a 25-year-old man has been arrested in connection with the incident.
He remains in custody for questioning.
In a statement, Avon and Somerset Police said: “There will be increased police presence at the scene while our enquiries continue and we’d encourage anyone who heard or saw anything which could help us to get in touch via 101 and quote reference 5216017838.”
