One person has been “seriously injured” and taken to hospital after being attacked at Thorpe Park.
The incident took place on a footbridge into the popular theme park in Surrey on Saturday, with guests locked down inside the resort as police attended.
The male victim’s injuries are not life-threatening and the incident is not being treated as terrorism, it is understood.
Two men have since been arrested on suspicion of attempted murder.
A father who is at the popular attraction with his kids said he saw paramedics attending to a patient on a bridge and "blood all over the floor".
One mother said her daughter is at Thorpe Park and has been told she can't leave.
Some reports claim people have since been allowed to exit the park - but say that there are huge queues to get out.
Security teams at the amusement park carry out bag searches and regularly patrol the premises.
Images on social media showed crowds of park-goers gathered together as several police officers attended the scene.
In a statement, Thorpe Park said its on-site medical staff provided emergency first aid and said it was helping police with its inquiries.
The statement said: “On Saturday, an incident took place at the Thorpe Park Resort where a guest was seriously injured by another guest.
“The resort’s security team and fully trained medical staff were at the scene within minutes and administered immediate care until emergency services teams arrived.
“The police were alerted immediately and the resort is co-operating fully with their investigations.
“The health, safety and security of our guests is our primary objective. We have an excellent security track record and have never had any incidents of this kind in over 40 years.”
A statement from South East Coast Ambulance Service says: "We were called to attend Thorpe Park in Surrey at about 4.50pm following reports of a person with an abdominal injury.
"On arrival, treated one person who was taken to a London hospital. We have no further information."
