Cyclists taking part in the 100-mile RideLondon event were diverted and delayed by hours after multiple air ambulances were scrambled to deal with an incident on the route.
Thousands were expected to take part in the ride, which set off from London on Sunday morning and followed a route into Surrey.
The “huge crash” resulted in hundreds of riders being temporarily halted.
“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 and 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.”
While cyclist Dom Cotton tweeted: “Bad RideLondon crash nr Ripley... Never seen so many air ambulances before.”
RideLondon organisers posted details of a diversion near Byfleet in Surrey, just before the Ripley blockage.