Police said they were called to Marsham Street where the Home Office (interior ministry) is located after a report of a man armed with a knife.
They found a man with knife injuries which were being treated as life-threatening. Another man was arrested at the scene on suspicion of causing grievous bodily harm.
A Reuters photographer saw a man with facial injuries and blood on his front being led from the Home Office building.
A police spokeswoman said it was too early to say whether the incident was linked to the Home Office but there was no suggestion it was terrorism-related.
“Enquiries are ongoing into the exact circumstances of the incident,” police said in a statement.
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