The PSNI said it is investigating what it called a “serious collision” between a scrambler motorbike and a pedestrian.
The pedestrian – a woman – was taken to the Royal Victoria Hospital.
Her condition at time of writing was described as “critical” by the Belfast health trust.
Police said the crash had happened at about 6pm in Colin Glen Forest Park, in south-west Belfast.
Police initially said one man was arrested. He is currently in police custody.
Later in the evening, they added that a second male had also been arrested.
Inspector James Murphy appealed to anyone who was in the Colin Glen area and may have witnessed this incident to contact police on the non-emergency number 101, quoting reference number 1041 19/07/16.