Police are searching for a 17-year-old boy who was reportedly seen entering a lake overnight.
Fairlop Waters in east London has been shut to the public by the emergency services.
The Metropolitan Police says it was called at 12.30am (July 4) to reports of a concern for safety.
Officers, including the marine unit, are in attendance along with London Ambulance Service and London Fire Brigade. A police helicopter has also been called in to help with the search.
Police say the boy's family have been informed.
Redbridge Council said in a statement just after 9.30am: "Following an incident on the lake at Fairlop Waters in the early hours of this morning, the park is closed to visitors at the request of the emergency services.
"At this moment we do not know when the park will be reopened."
Redbridge Council deputy leader Cllr Kam Rai tweeted: "Following what appears to be an incidence of misadventure on the lake at Fairlop Waters in the early hours, the park will be closed until further notice."
As of 1pm, Fairlop Waters remains closed.
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