An air ambulance landed in Trafalgar Square after an 81-year-old pedestrian was seriously injured in a motorbike crash.
The pensioner was rushed to hospital with a head injury as a result of the collision.
Paramedics rushed to the scene of the crash, which happened at the Piccadilly Circus end of Shaftesbury Avenue.
Police said they were called to reports of a man colliding with a motorbike, and roads around the landmark have been closed off.
The Met Police said: "Shaftesbury Avenue and Piccadilly Circus are currently closed as police and London Ambulance Service attend a collision involving a pedestrian and a motorbike.
"Police were called at about 11:20hrs on Sunday, 24 April to Shaftesbury Avenue.
"The 81-year-old pedestrian was taken to hospital and we await an update on his condition.
"Motorists are advised to take alternative routes where possible.
"The motorbike stopped at the scene.
"There have been no arrests and enquiries are ongoing."
A London Ambulance Service spokesperson said: “We were called at 11:20am today (24 April) to reports of a road traffic collision at the Piccadilly Circus end of Shaftsbury Avenue.
“We sent a number of resources including an ambulance crew, a paramedic on a bicycle, a clinical team leader and an incident response officer. We also dispatched medics from London’s Air Ambulance.
“We treated a man at the scene for a head injury and took him as a priority to a major trauma centre.”
Hundreds of people looked on as the air ambulance landed at the busy square in the heart of the capital.
A fundraiser for charity London Air Ambulance, which responded to the emergency, posted on Twitter : "In all my years of fundraising for London Air Ambulance and in working in the West End this is the first time I've been passing Trafalgar Square as our lifesaving team lands.
"I felt proud and told the people around me what was happening and how it's funded by charity."
