Sussex Police, the South East Coast Ambulance Service (SECAmb) and East Sussex Fire and Rescue Service were all called to the scene just before 3pm.
A SECAmb spokesperson has since confirmed that a teenager has been taken to hospital with a leg injury, and others were 'shaken but uninjured'.
The spokesperson said: “South East Coast Ambulance Service has responded to an incident on Brighton Pier today in which it was reported, at approximately 3pm, that a piece of a ride had come loose.
"A male teenager has been taken to hospital with a leg injury. Others were shaken but uninjured. We are now clear from the scene.”
A Sussex Police spokesperson said: "Police and other emergency services are attending Brighton Palace Pier after a fairground ride was reported to have failed at about 3pm on Monday.
"Initial indications are that four people have sustained injuries that are not thought to be serious at this time.
"There are currently no further details."
An East Sussex Fire and Rescue Service spokesperson said: "We are attending an incident at the end of the pier.
"We were called at 3.06pm. We're assisting SECAmb."
Anne Ackord, chief executive of the Brighton Pier Group, said: "An incident occurred yesterday when part of one of our rides (Air race) became detached during the ride cycle and a passer by was hit by the part of the ride.
"The ride has not experienced any issues before and we have been in immediate contact with the manufacturer to ask them to send a team of engineers to investigate. The engineers will arrive tomorrow when they will cooperate fully with any HSE (Health and Safety Executive) investigation.
"We take the safety of our customers extremely seriously and are pleased to hear from the young man concerned that he has minor injuries.
"Until we have the result of the investigation we cannot comment further other than to reassure our customers that safety is our prime concern and that this incident is previously unheard of on any Air Race throughout the world and as such exceptionally rare."