An incident has sparked panic at Adventure Island in Southend, Essex, with people 'running and screaming' through the theme park.
The attraction has reportedly been closed over the incident, although it is not yet clear what exactly happened.
One witness said a huge brawl involving up to 40 people broke out while people were queuing for a rollercoaster.
They said some of the group were armed with knives and security were powerless to stop the violence.
The eyewitness told The Mirror: "A fight broke out in the queue on the rollercoaster opposite the main ticket office.
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"Security guards ran to the incident and there was about 30 to 40 people fighting.
"Security couldn’t contain the incident.
The suspects had knives. Everyone was told to evacuate or stay inside the centre area.
"I and a number of other people called 999. There was a huge police attendance.
"One father got separated from his daughter - she was eventually found safe and well."
One Twitter user also said the theme park shut down after a fight over a ride.
The user shared a video showing a group of police officers gathered together at the site.
A different person wrote: "How can I go to adventure island and a fight breaks and security starts getting beat up."
Another social media user said: "Not sure what's happening we have been evacuated out of Adventure Island due to a code Red.
“There's loads of Police and other Emergency services here."
One person said the incident sparked a 'stampede', with 'everyone running and screaming and kids getting lost'.
A bus company has confirmed a service in the area has been stopped due to a police incident in Southend.
EnsignBus tweeted: "Due to a Police incident, buses are unable to serve the Kursaal bus stops for the rest of the day. Please use the Southend Pier bus stop."
Essex Police has been approached for comment.
