British Transport Police are advising people to avoid the London Bridge area, while the incident is being investigated.
The ambulance service says multiple resources have been sent to the scene.
Pedestrians have reportedly been hit by a van on London Bridge in Central London.
An eyewitness told the BBC several people were injured.
Armed police officers and paramedics are now working at the scene, in what Transport for London describes as a "major police incident".
Stay with us for updates on this fast-developing story.