A "nasty crash" between Pyrford and Ripley, Surrey has brought the annual RideLondon cycling event to a halt.
Cyclists queued for up to an hour for the crash at around the 40 mile mark as paramedics and an air ambulance attended the scene.
Route organisers were eventually forced to open an alternative route from West Byfleet to Ripley to allow riders to get through but many cyclists had already past the diversion point.
Locals reportedly handed out water to stranded cyclists while they waited on the route.
Cyclist Sally Hughes said she had been stopped at the front of the huge queue.
She said: "Hope it's not too serious - sounds nasty".
Another, David Long, said: "Big crash at Pyrford, been waiting for an hour - hope all involved are OK".
Tim Sculthorpe reported that a police helicopter could be seen hovering overhead.
A spokeswoman for the South East Coast Ambulance Service said the rider had crashed into a tree in near Pyrford and was airlifted to St George Hospital in Tooting, south London.
A second rider also came off his bike in the Thames Ditton area.
She said: "He is suffering a head injury and will be transferred to one of the London Hospitals."
The ride reportedly began moving again at around 10:45am after organisers opened the contingency route.
At around 11:15am the route was reopened.
Rider Sharif Hossain told The Independent cyclists were beginning to move again.
He said they were "crawling, but moving".
Additional reporting by PA
