Emergency services were called to Westminster after a man reportedly fell from the bridge.
London Air Ambulance crews arrived shortly after 12.03pm after a man fell around "15 feet onto concrete".
A spokeswoman for the Metropolitan Police said: “Police were called to reports of a man having fallen from Westminster Bridge onto concrete steps close to Speaker's Green. "
Witness Michael Wilkinson, who works in an office near the scene, told getwestlondon: "The helicopter landed right in the middle of Parliament Square with tourists backing up to escape the air stream.
"Then the paramedics ran as fast as they could through the grounds of Parliament. Police held back traffic and opened the gates to the site.
"We were watching from our offices above. It is not something you see every day and of course we were shocked after hearing what had happened."
A spokesman for London Ambulance Service said: “We were called at 12.03pm to an incident at Westminster Bridge involving a man falling approximately 15 feet onto concrete near the Houses of Parliament.
“We sent a number of resources including an ambulance crew, our hazardous area response team, an advanced paramedic in a car and London’s Air Ambulance.
“We treated the man at the scene for a serious head injury and took him by road ambulance to a major trauma centre."