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The coastguard helicopter was seen scouring the River Medway after a person jumped from a bridge.
A huge search and rescue operation was launched after people reported seeing a man plunging into the water from the Medway Bridge.
Traffic ground to a halt on the London-bound M2 between junctions 3 for Chatham and 2 for Strood, which was completely closed until around 5.20pm.
Police, ambulances, fire crews and lifeboats were seen by the water at Medway Valley Leisure Park, near the Cineworld cinema.
The police helicopter was also seen hovering over Cuxton and the Medway Valley area.
A police spokesman said: "Kent Police was called at 4.30pm to reports a man was seen jumping from a bridge on the M2 that goes over the River Medway.
"The London-bound section was closed until 5.20pm while officers carried out a search.
"The road has been re-opened and the search continues below the bridge."
It is not yet known if the man has been found, or what his condition is.