Armed police and an air ambulance were called to a disturbance in Deeside.
A footbridge near Bridge Street in Shotton was taped off as emergency services descended in large numbers after 6pm.
A North Wales Police spokesman said: “Police are appealing for witnesses following an incident on open ground near Bridge Street before 6pm where a male received serious injuries.
“The man has been transferred to hospital by air ambulance.
“Call 101 quoting reference number V140551.”
Armed officers were seen alongside seven police vehicles, Welsh Ambulance Service emergency vehicles and the Wales Air Ambulance which left the scene at just before 7pm.
A resident near Bridge Street said: “The air ambulance was here for 20 minutes and armed police came from the Shotton railway side from behind the old Corus club.”