Police have confirmed at least two people were injured after as many as 600 people turned up at an illegal rave in Borehamwood overnight.
Officers are investigating a suspected double stabbing which took place at an unlicensed music event off Elstree Way.
Hertfordshire Constabulary says two teenagers suffered injuries - believed to have been caused by a knife - at the event, which was attended by hundreds.
Police say they became aware of a music event taking place at a warehouse near York Way shortly before midnight.
Further support was called in and police say they moved in to shut the party down when at least 200 were taking part in the rave.
A Snapchat video shows people packed inside a building partying.
Police say that while they were in the process of shutting the party down, they became aware of at least two people injured.
Hertfordshire Constabulary is saying a "violent incident" took place, which left at least two people hurt.
Two teens suffered injuries to their hand and head and they were taken to hospital.
They are not in a life-threatening condition.
Police have not ruled out that more may have been injured.
Video shows several police vehicles in Elstree Way, including police vans, while there was also a police presence in Studio Way.
Ambulances also attended.
Police van in Elstree Way
Police say many of those who attended came from outside of Borehamwood, with dozens seen walking through the town centre from Elstree & Borehamwood station.
Chief Inspector Frankie Westoby said: "This was a large event with up to 600 people – many from outside Hertsmere – travelling to the area to take part. We showed that, with support of partners, we were able to mobilise resources quickly to stop most of them accessing the site and to close the event down.
"Sadly on this occasion a violent incident occurred resulting in two teenagers suffering injuries. We would appeal to anyone who witnessed this or who has any information about it at all, to contact us as soon as possible.
"There was also information that other people may have also been injured but who left the scene without presenting themselves to the emergency services. If anyone else was hurt, we would urge them to come forward and speak to us as soon as possible.”
People seen walking towards where the event was taking place
CI Westoby added: "I would like to take this opportunity to remind people that unlicensed music events are illegal and by their very nature there are numerous risks involved in attending.
"As well as the continuing threat of COVID-19, they are often held at sites that are unsuitable for large gatherings and organisers often fail to take appropriate steps to ensure people’s safety. Our advice is to stay away."
Earlier in the night, police had tweeted: "We are aware of an unlicensed music event expected to take place in #Borehamwood tonight. If you are currently travelling to the event, please do not attend.
"Police officers are currently in attendance at the location and are closing the event down."
Anyone with information can report information online at herts.police.uk/report or call 101 or call Crimestoppers anonymously on 0800 555 111 or visit crimestoppers-uk.org.
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