The Cut in Waterloo was taped off by police on Thursday morning while officers investigated a fight on Wednesday evening.
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.
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