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People have been asked to stay away from a stretch of beach while the emergency services deal with an "ongoing incident".
Essex Police said they were called to the seaside town of West Mersea, near Colchester, on Wednesday evening.
In a Facebook post, the force said: "While we remain on scene, we would ask the public to avoid the Seaview Avenue and beach area.
"We appreciate that this will cause some local issues, but would ask for your patience and understanding as we may be there for some time yet."
An RNLI spokeswoman confirmed its West Mersea lifeboat was launched earlier this evening and its lifeboat from Clacton was also sent to the emergency.
She added she did not know the nature of the incident at this stage.
The BBC has contacted Essex Police, HM Coastguard and the East of England Ambulance Service for more details.
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