Two cyclists taking part in the 100-mile RideLondon event have been airlifted to hospital after two separate crashes.
One man is in a serious condition after hitting a tree and another is being treated for a serious head injury after falling off his bike, South East Coast Ambulance Service said.
The man was taken by air ambulance to St George’s Hospital in Tooting, London.
The second crash took place at around 10.55am near Thames Ditton and he was also treated by air ambulance paramedics before being taken to a London hospital.
Thousands were expected to take part in the ride, which set off from London on Sunday morning and followed a route into Surrey.
Organisers tweeted: “Due to an incident that required an air ambulance to attend, riders in the RideLondon-Surrey 100 have been held on the route in Surrey.
“Planned contingency route from West Byfleet to Ripley is being opened & riders will be directed to that route. More info when available.”
Some participants and spectators on social media suggested there had been a prolonged hold-up.
Maria David said on Twitter: “Been stuck at Pyrford feed station about 45 minutes. Apparently air ambulance on the road up ahead.”
Dom Cotton tweeted: “Bad RideLondon crash nr Ripley… Never seen so many air ambulances before.”
Dominic Harris, who is taking part in the event, tweeted: “RideLondon participants waiting patiently in Pyrford following what is understood to be a medical incident ahead.”
Organisers released a picture showing the diversion, which began at West Byfleet and looped away from the original route before rejoining it at Ripley.
A spokeswoman for RideLondon said the diversion would extend the course by about one mile.
Shortly before noon, RideLondon tweeted: “The route from West Byfleet to Ripley has been now been re-opened to riders in the Prudential RideLondon-Surrey 100. The diversion route is no longer in use.”