The Central Line station was closed to eastbound trains at around 3.35pm, Transport for London (TfL) said.
One passenger on a westbound train that stopped at Marble Arch station described the “terrifying” moment the doors opened to “loads of screaming people”.
Another passenger reported seeing “about a dozen medics” or TfL staff treating a person in the station.
A third passenger onboard a train passing through the station posted video on Twitter showing a forensics team working on the platform.
London Ambulance Service said paramedics had treated a man for a head injury and rushed him to a major trauma centre.
Marble Arch station reopened shortly before 6pm.
The Central Line twitter account posted: “The @centralline station has now opened fully, following a police investigation.”
Police have launched an appeal for information after officers responded to reports of an assault in the station.
A statement from British Transport Police (BTP) said a man had been detained.
A passenger travelling home from Tottenham Court Road witnessed the evacuation of Marble Arch station from the westbound platform.
Abi, 31, told getwestlondon: “It was incredibly scary, it really was terrifying – I’ve lived in London 11 years and never seen anything like this before.
“I was on the westbound train which pulled into Marble Arch. The doors opened and pretty soon after there was loads of screaming coming from the other platform and lots of people ran away from the commotion.
“I couldn’t hear anything there was so much screaming and the alarm went off on the train, which I assume is because someone had pulled it.”
Transport for London said on Twitter: “This station is closed to eastbound trains only while the police carry out an investigation. Westbound trains are stopping normally.”
Anyone with information which could help the investigation is asked to text BTP on 6101.
You can also call 0800 40 50 40, quoting 344 of 27 April