Police and ambulance crews were called to an incident at Richmond Bridge.
An eyewitness said she saw paramedics looking over the bridge into the water at around 12.30pm yesterday.
The witness said: “I was coming over the bridge and there were two ambulance people on the right-hand side of the bridge, and then they ran over to the other side of the bridge.”
A spokeswoman for London Ambulance Service (LAS) said a person had been taken ill.
She said: “We were called there at about 11.45am by the police to reports of an incident. We sent one ambulance crew to the scene, and the last update was that we are still on the scene on stand-by from the police.
“We haven’t treated any patients.”
A spokeswoman for Richmond police said officers had been searching for a missing person and the bridge was one of "many locations" searched. She added that no one had been found, however she could not confirm if the person was still missing.
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