Lifeboat and police at scene of West Mersea incident
Lewis Adams/BBCPeople 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, and have been seen searching the coastline and water.
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."
Lewis Adams/BBCAt the scene: BBC reporter Lewis Adams
"Part of the beach at the bottom of Seaview Avenue in West Mersea has been cordoned off and police are focusing their on-land inquiries on part of the sand where there are windbreakers in place.
This is a spot that gets really busy when it's hot, being the main beach on what is seen as a pretty exclusive island on the Essex coast, south-east of Colchester.
But tonight it is busy for a different reason. There has been a huge presence of police, paramedics and water rescue workers in the area - and they remained there as it got dark.
A lifeboat could be seen launching at one point and searching the water. Police officers are also searching the shoreline with torches as the light fades."
Lewis Adams/BBCAn 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.
A spokeswoman for the East Anglian Air Ambulance said it had also sent a helicopter to West Mersea, but she had no further information.
The BBC has contacted Essex Police, HM Coastguard and the East of England Ambulance Service for more details.
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