A man is in a critical condition after he was stabbed on Westminster Bridge in central London, the Met Police has said.
Emergency services were called at about 10:45 GMT to reports of a fight on the bridge.
London Ambulance Service and the air ambulance also attended and took him to hospital.
The Met said three people had been arrested on suspicion of attempted murder while another arrested for affray.
Two of those arrested were taken to hospital with minor facial injuries, the Met added.
It is understood the incident is not being treated as terror-related.
Westminster Bridge is closed each way as are the roads around it while the police carry out investigations.
Buses including 11, 12, 148, 159, and 453 are on diversion.
