Nine specialist bikes worth a total of around £30,000 have been stolen from the National Mountain Bike Championships.
The mountain bikes were stolen some time between 2300 BST on Saturday and 0700 BST on Sunday at the event in Margam Park near Port Talbot.
Police said the bikes were worth between £3,000 and £4,000 each.
The third round of the championships was held at the park over the bank holiday weekend.
Police are appealing for information about a silver van which was seen entering the park at 2330 BST on Saturday and leaving at 0500 BST on Sunday.
Acting Ch Insp Jeff Handley, of South Wales Police, said: "There is no doubt that this event has been targeted by a group of organised thieves.
"All the victims were visiting our area to enjoy the fantastic mountain biking facilities on offer and for them to return home having been a victim of a crime must be sickening for them."
Anyone with information is asked to contact South Wales Police on 101 or Crimestoppers anonymously on 0800 555 111.