A man suffered serious injuries after an altercation on the concourse at Waterloo station, police said.
A section of the station was cordoned off following the incident at around 8.45pm on Saturday.
British Transport Police said one man has been arrested on suspicion of affray and a second on suspicion of GBH.
Officers had previously said they were seeking a second suspect, who he was since arrested and both men have been released under investigation.
London Ambulance Service also attended the scene before taking the man to hospital for urgent treatment.
A spokesman for BTP said they were called "following reports of an altercation that resulted in a 44-year-old man sustaining serious injuries.
"He was taken to hospital and is currently in a serious but stable condition," officers added.
"A 28-year-old man was arrested on suspicion of affray and a 34-year-old man was arrested on suspicion of GBH. Both have been released under investigation."
Anyone with information which can help the investigation is asked to call 0800 40 50 40 or text 61016, quoting incident 622 of October 20.