Witnesses said the victim was shoved beneath a Northern Line service at Kentish Town station as shocked passengers looked on in horror.
He survived but suffered serious injuries and was emergency air-lifted to hospital to be treated by medics.
British Transport Police confirmed the man was struck by the Tube at around 11:45 this morning.
BTP said it was treating the incident as "suspicious" and had received reports "the man may have been pushed".
A Tube worker told the Evening Standard: "I have never seen anything like this it's like something out of a film. We are in a crime scene and trying to deal as best we can."
One witness described the moment it happened, urging fellow passengers to "stay safe".
I feel physically sick. Someone has just been pushed under the train at #kentishtown underground station. Please everyone stay safe.— Verity Slattery (@VERITYSWORLD) December 2, 2015
While another said the train driver had sounded 'very panicked' on the tannoy immediately after the incident.
Was on the train at #kentishtown - driver on tannoy sounded v panicked as they kept us on board until evacuating— Jade Bowyer (@jadebowyer) December 2, 2015
Several stations on the same line, including Highgate and Archway, were closed in the aftermath while police tried to assess whether the victim was pushed or not.
London Ambulance Service said in a statement: "We treated a man at the scene and took him as a priority to a hospital in central London, with a doctor from the air ambulance on board."