Armed police swarmed a station in Mile End after a man was stabbed.
Officers from the Metropolitan Police were dispatched with armed officers to Mile End underground station at 2.27pm on June 13, following reports of a man with a knife.
At the scene, a man was discovered to be suffering from life-changing stab wounds and was taken to hospital.
The same man has also been arrested on suspicion of possession of a bladed article in a public place and affray.
They made a second arrest, detaining a 17-year-old boy on the same suspected offences and on suspicion of grievous bodily harm.
He remains in custody.
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Concerned social media users took to Facebook to ask what was going on.
One user wrote: "Something happened at Mile End station as well six armed police there."
Witnesses, or anyone with information is asked to contact British Transport Police by texting 61016 or calling 0800 405040 quoting reference 335 of 13 June.
Alternatively, call Crimestoppers anonymously on 0800 555 111.
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