Passengers at Kings Cross were cleared out to avoid crushing, and images posted online show the station gates closed.
A large police presence is currently on the scene, leading to speculation of an emergency situation.
But Transport for London said the station was exit and interchange only on account of it being too busy.
A Transport for London spokesman confirmed to Express.co.uk that the closure was due to overcrowding.
In the last few moments TfL announced the station had been reopened.