An 11-year-old girl has been sexually assaulted in Llanbradach Park.
Police said the attack, which is described as serious, happened near the village library on Monday at noon and are appealing for people in the area at the time to come forward.
A 19-year-old man from the Pontypridd area has been arrested in connection with the incident and is currently being questioned.
Detective Superintendent Geoff Ronayne, who is leading the investigation said: “The young girl and her family are currently being supported by specialist officers following this horrific incident.
“We are appealing for anyone who was in the area of Llanbradach Park between the hours of 8am and 2pm, on Monday January 4, to come forward as they could hold vital information that could assist our investigation.
“In addition to this if you saw anyone acting suspiciously in the area of the park in the last few weeks please contact us.
“The serious sexual assault of an 11-year-old girl is an extremely rare occurrence in Gwent. We have set up a dedicated team to investigate this incident. The area has been cordoned off, house to house enquiries are underway and local police patrols have been increased.”
The attacker was male, wearing a black hooded top with a grey back on it, army-style trousers and brown shoes.
Police believe he had a bicycle with him, possibly a mountain bike or BMX.
Anyone with information should contact police on 101, or call Crimestoppers anonymously on 0800 555 111.