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Emergency services were called to an incident near the River Medway.
Police, fire, paramedics and the coastguard were at the scene in Canal Road, Strood.
Police were called at 10.27am to concerns for the welfare of a person reported to have fallen in the river.
The coastguard helicopter was seen circling the area at 11am but left shortly after.
The air ambulance soon arrived and landed nearby but left just before midday.
At its height, around 20 emergency vehicles were at the scene and a lifeboat was in the water.
A KentOnline reporter at the scene said: “The search is outside The Boat House pub near the old Soviet submarine.
“The search and rescue team and paramedics were on the river bank with a stretcher but they have since left the scene and got into an ambulance to leave.
“The ambulance and police remain on the scene and a search and rescue boat is currently still circling in the river.
“The air ambulance has also left. It’s still not clear what has happened.”
A Coastguard spokesman said: “We were alerted at around 10.30am, rescue teams from Sheppey and Medway, an RNLI lifeboat from Sheerness and a coastguard helicopter have been sent to assist.”