Brazilian police have been questioning a man accused of driving his car at high speed through a crowd of cyclists, injuring at least 12 of them.
Video of the incident in the southern city of Porto Alegre shows bikes and riders flying through the air as other cyclists scream in panic.
The driver said he accelerated because cyclists were banging on his car and he feared he would be attacked.
The cyclists were holding a regular event to promote the use of bikes.
The incident happened on Friday evening but came to national prominence in Brazil on Monday when television networks began broadcasting amateur footage that had been posted on the internet.
No-one was killed and the injured have all been released from hospital.
The driver, Ricardo Neis, 47, fled the scene of the incident but was brought in for questioning after his abandoned car was found.
Police said he could face a charge of attempted homicide.
Cyclists in the demonstration said no riders had threatened the motorist.
The riders were part of a movement known as Critical Mass, which holds mass bike rides in cities around the world to demand more respect for cyclists and is sometimes accused of deliberately blocking traffic.