The Cut in Waterloo was taped off by police on Thursday morning while officers investigated a fight on Wednesday evening.
A Met spokesman said that police were called to The Cut by London Ambulance Service just after 8.45pm on Wednesday to a report of two men fighting.
Two men were treated at the scene before being taken to hospital.
One of the men’s condition has been assessed as life-threatening. The other man’s condition is not believed to be life-threatening.
A crime scene has been put in place and police are continuing to make enquiries. No-one has been arrested in connection with the incident.