The Met confirmed that officers were called just after 3pm on Friday to reports that a boy, 16, got into difficulty in the water.
The boy’s next of kin has been informed and is currently being supported by specialist officers.
Emergency services including Met Police, London Fire Brigade and London Ambulance Service, are currently co-ordinating a search.
Teams from Lewisham Council are also supporting officers on site, as the search continues into Friday evening.
According to witness reports, emergency services were seen searching the water with raft boats this afternoon.
A spokesperson for Met Police said: "Police were called at 15:08hrs on Friday, 04 April to Beckenham Place park.
"A 16-year-old boy is believed to have got into difficulty in the water and is now missing.
"Emergency services are currently co-ordinating a search.
"The next of kin has been informed and are currently being supported by specialist officers. The park has been evacuated to facilitate the search."
A London Ambulance Service spokesperson said: "We were called at 3.02pm this afternoon to reports of a person in the water.
"We sent resources to the scene, including an ambulance crew, an incident response officer and members of our hazardous area response team.
"We are still at the scene working alongside our emergency services partners."
More to follow...