One person was “seriously injured” and taken to hospital amid reports of a stabbing at Thorpe Park.
The theme park in Surrey said on-site medical staff were at the scene “within minutes” after an incident between two visitors.
The victim is understood to be male and is in hospital with non life-threatening injuries.
It is understood the incident is not being treated as terror-related.
Visitors were locked down inside the resort while police attended. Pictures on social media showing crowds bunched up together after the incident.
South East Coast Ambulance Service said they were called to the park at about 16:05 BST following reports of a person with an abdominal injury. They treated one person who was taken to a London hospital.
In a statement, Thorpe Park said: “The health, safety and security of our guests is our number one priority and we have never had any incidents of this type in over 40 years of operating.”