Uxbridge station was forced to close after a 'large number of people' descended on the platform around midnight on Saturday night.
It is unclear at this stage who these people were or what they were doing at the station.
Nigel Holness, operations director for the Piccadilly Line, said: “Uxbridge Station on the Piccadilly Line was closed at 12.05am on Sunday morning after a large number of people descended on the station together and a female member of staff was assaulted, taken to hospital by ambulance and discharged.
"The station was made exit only at 12.24am until the end of the train service which was not interrupted by the incident. BTP and Metropolitan Police attended the incident.”
People took to Twitter to discuss what had happened after the Metropolitan line's account @metline announced the closure.
The Metropolitan and Piccadilly lines serve the station, which is in Uxbridge town centre.
The station will close for almost a month from July 19 as track repairs are carried out.
This section of the London Underground also found itself at the centre of this week's View from the Balcony blog, in which Scott discusses the 24 hour tube - or lack of it for Uxbridge commuters.
