Officers from the Met Police were called to the scene of the incident in Parsloes Park at 7pm on March 14.
In the park, the victims had allegedly been threatened with a gun.
The robbery is one of “multiple reports” received by police concerning the park, leading them to increase foot patrols in the area.
The victims were not injured in the incident and there have been no arrests at this stage.
A Metropolitan Police spokesperson said: “Police were called to a reported robbery in Parsloes Park, Dagenham at 19:00hrs on Friday, 14 March, where the suspect allegedly threatened the victims with a firearm.
“No one was injured in the incident. While no arrests have been made at this stage, an urgent investigation is underway.
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“We are aware of multiple reports of robberies in Parsloes Park and have increased foot patrols in the area as a result.”
Anyone with information or who witnesses an incident should contact police on 101, quoting reference 6489/14Mar.
Incidents that are an emergency should be reported by calling 999.
