The London Ambulance Service says it has treated at least 10 injured people on Westminster Bridge after a vehicle hit pedestrians.
It says the first ambulance arrived within six minutes of the first call at 2:40 p.m. (1440GMT).
Ambulances, an air ambulance and a Hazardous Area Response Team were all sent to the scene.
One woman has been confirmed dead, and a body was seen lying in the yard of Parliament, where a knifeman stabbed a police officer and was shot by police.
People began leaving the Houses of Parliament about two hours after the incident.
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