A man, thought to be aged about 30, was taken to hospital on Friday evening after he was assaulted at the Fire Station bar in Waterloo Road.
A Scotland Yard spokeswoman told the London SE1 website that police were called to The Fire Station at about 6pm on Friday evening.
The Met spokeswoman added that officers from Lambeth Police are investigating the incident and monitoring the condition of the victim who is being treated in hospital.
The Fire Station was in the news last week when Lambeth Council's licensing sub-committee moved to ban drinkers from congregating on the pavement outside the bar.
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