Armed police swooped to Blackheath Common on Tuesday evening to a number of assaults and a reported stabbing.
No victim for the stabbing was found and the fair’s organisers dismissed reports of an acid attack on social media as false.
Blackheath Bank Holiday Funfair Organiser Joe Noyce said “I’d like to thank the police for their swift action in dealing with the large group of youths seeking to cause disruption and unrest in this area of Blackheath Common.
“As a family business ourselves, we want to ensure families coming to our fair feels safe, so we have enlisted additional onsite security, and are grateful by police reassurances of additional patrols in the local area.
The statement added: “Social media rumours of stabbings or the use of acid at the fairground are false, and no one required first aid by our onsite team.”
Footage on social media shows dozens of young people charging through the fair.
A police spokesperson said: “Police are dealing with reports of antisocial behaviour and a number of assaults at Greenwich Park.
“A stabbing has also been reported; no victim has been found.”
They added: “Three people have been arrested for possession of an offensive weapon and related offences.”
Officers have put in place a Section 60 order until 5pm Wednesday which gives officers extra search powers.
A witness Sean Ash, who had driven to the Common to take his children to the funfair, said there were hundreds of children on the green space and scenes of disorder.
“When I arrived, we saw a police van, and there was hundreds of young people, all aged about fifteen, sixteen,” he told The Standard.
“My son was in the back seat, and he said, ‘They’re hitting the police’. A police chopper came over the top, they’re still there now.”
The leader of Greenwich Council said he was aware of the disorder and issued an “urgent appeal” to parents in the borough to make contact with their kids if they did not know where they were.
“If your child is in the #blackheath area – or you don’t know where they are – please make urgent contact. Make sure they go home asap,” said Dan Thorpe.
The area’s MP, Matthew Pennycook, also urged locals to avoid the area.
“The police are deploying extra resource,” he said. “Please avoid the area. If you believe your child is at the funfair or in the area, please ask them to return home immediately.”
Organisers said additional safety measures will be in place when the fair reopens at 1pm on Wednesday including; extra security officers, enhanced safety checks at entrances and teenagers will need to be with an adult to gain entry.