Two cyclists taking part in the 100-mile RideLondon event have been airlifted to hospital after 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.
Hundreds of riders taking part in the annual event were held up for prolonged periods as emergency services responded to the first crash at 9.05am near West Byfleet, Surrey.
The first rider was taken to St George's Hospital in Tooting, south London. It is not clear if he hit the tree before or after he fell off the bike, an ambulance spokesman said.
The second crash took place at around 10.55am near Thames Ditton, Elmbridge, and the cyclist was also treated by air ambulance paramedics before being taken to a London hospital.
Thousands took part in the ride, which set off from the Olympic Park in east London on Sunday morning and followed a route into Surrey before looping back to finish on the Mall, central London.
A diversion was put in place for several hours and a backlog of cyclists formed, prompting organisers to cut the route by 26 miles to get participants to the finish line on time.
Organisers released a picture showing the diversion, which began at West Horsley and cut out a quarter of the original route through the county before rejoining it near Leatherhead.
The new route missed out Box Hill, which has become a popular attraction for cyclists after it featured prominently in the London 2012 Olympic cycling road race.
RideLondon tweeted: "Following the incident near West Byfleet when a rider required an air ambulance, a backlog of riders built up on the route.
"In order to ensure all riders get to the finish line within the required 8.5 hours, a planned diversion route has been opened at Mile 44 to divert those riders through to Mile 70 (taking out Box Hill and rejoining the route before Leatherhead)."