Officers from the British Transport Police were on patrol at Dagenham Heathway underground station on January 9.
It forms part of an operation in response to preventing violent crime and robbery on the rail network.
During their shift, they stopped a teenage boy who was seen “acting suspiciously” at around 5.45pm.
A search was carried out by officers and a knife was found, according to police.
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The teen was then arrested on suspicion of possession of a bladed article.
Footage shared online from January 9 showed a large crowd forming outside the entrance to the underground station on the Heathway, being blocked by a barrier.
A large team of officers could be seen through the barrier surrounding a person.
