Emergency services rushed to the scene after a person fell from London Bridge, police say.
The person was treated at the scene by paramedics before being taken to a Major Trauma Centre as a priority in the early hours of this morning.
A main road was closed and police advised people to find alternative routes where possible.
City of London Police tweeted: "Upper Thames Street is currently closed west bound near Fish Street Hill due to an ongoing incident involving a person who has fallen from London Bridge."
Some transport services were being diverted amid the incident.
TfL Bus Alerts tweeted: "Route 344 is diverted due to a police incident which has caused Lower Thames Street to be closed westbound near London Bridge.
"Services are diverted via London Bridge, Southwark Street and Southwark Bridge Road."
Have you been affected by this incident? Email webnews@mirror.co.uk
A London Ambulance Service spokesperson told Mirror Online: "We were called at 5:53am today (05 May) to Upper Thames Street, City of London to reports of an incident.
"We sent a medic in a car, an incident response officer and an ambulance crew to the scene.
"We also dispatched London's Air Ambulance by road.
"We treated a person at the scene and took them to a Major Trauma Centre as a priority."
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