Police were still trying to break up an illegal rave in Yate, near Bristol, more than 14 hours after being called to the site.
Avon and Somerset Police were informed of the event at a warehouse in Millbrook Road around 10.30pm on Saturday.
Officers arrived within 12 minutes of the call and found several hundred people and a number of vehicles. A dispersal notice was issued requiring them to leave.
Units from the dedicated team dealing with unlicensed music events were deployed and roads were closed, but large numbers continued to arrive on foot.
"A number of the people being prevented from entering the premises became hostile towards the police. Items, including lit spray cans and bottles, were thrown at police, some of whom were injured but remained on duty," the force said.
"A secure cordon was put in place by 4am, but it is estimated that approximately 500-700 people were on site. While a number of people have left, it is believed a significant number remain within the premises."
Western Power attended and cut electricity to the building but an alternative source was found by the rave organisers.
Police asked for help from neighbouring forces to try to close down the event, and two people have so far been arrested.
Police are dealing with an unlicensed music event attended by hundreds of people in South Gloucestershire.
At about 10.35pm we received a call alerting us to the event at a warehouse in Yate. Officers arrived within 12 minutes and found the event underway. pic.twitter.com/Zyr0SMDpKT
Chief Inspector Mark Runacres said: "This has been an incredible challenging operation with a large number of people in a confined space and several officers being assaulted.
"We are working with neighbouring forces and the ambulance service to look to bring this event to a close in a controlled way that does not jeopardise the safety of people in the area.
"We're sorry for the disruption caused by the selfish actions of the organisers of this event and those attending. This unlicensed event was organised in secret, despite people being fully aware of the ongoing risk to public health due to Covid-19.
"Their actions were wholly irresponsible and I am sure will disgust the overwhelming majority of people who are making huge sacrifices to limit the spread of the virus.
"Our investigations team will be involved in reviewing officers' body-worn footage and other inquiries as we seek to take appropriate action against those responsible.
"We'd ask anyone with evidence and information that can help, to call 101 and quote log number 1369 of October 31."
Pictures of the week: October 25 - 31
Pictures of the week: October 25 - 31
EMBARGOED TO 0001 THURSDAY OCTOBER 29 EDITORIAL USE ONLY Sculptures of �invisible women� are unveiled in London to announce the launch of Generation Menopause: The Invisibility Report, a white paper evidencing the experiences of menopausal women in the UK today.
Rain falls in the Old Market Square in Nottingham ahead of the region being moved into Tier 3 coronavirus restrictions on Thursday. The very high level of restrictions includes a ban on social mixing both indoors and in private gardens, pubs and bars closing unless they can operate as a restaurant, and residents are advised against overnight stays in other parts of the UK and they should avoid travel where possible in and out of the area, unless it is for work, education or caring responsibilities.
Celebrities in Halloween Costumes - File Photo by: zz/KGC-49/182/STAR MAX/IPx 2016 10/31/16 Lindsay Lohan (dressed as Harley Quinn) arrives at a Halloween Party in London, England, UK.
LONDON, ENGLAND - OCTOBER 29: A passenger on the tube in central London on October 29, 2020. The coronavirus pandemic has reached a âcritical stageâ in England with 96,000 new infections in a single day and infections doubling every nine days, according to the Imperial College London (ICL) on Thursday. Health Secretary Matt Hancock announced that 16 more areas in England will move into Tier 2 restrictions, the second highest level, beginning Saturday. (Photo by Kate Green/Anadolu Agency via Getty Images)
Actors and performers protest against coronavirus restrictions as part of a week of protests in Parliament Square in London, Thursday, Oct. 29, 2020. The British government is under pressure to develop a national strategy to combat the resurgence of the COVID-19 pandemic and “rescue Christmas’’ as scientists warn that the number of people hospitalized with the disease could almost triple by the end of next month unless something more is done now. (AP Photo/Frank Augstein)
A man wears a face mask as he passes the London Eye in London, Thursday, Oct. 29, 2020. The International Monetary Fund has downgraded its growth forecasts for the British economy for this year and next, in its annual assessment published Thursday, following an acceleration in the number of coronavirus infections over the past couple of weeks.(AP Photo/Frank Augstein)
A general view during The Cotswold Racing At tipstersempire.co.uk Mares' Handicap Hurdle at Chepstow Racecourse.
A woman and remote controlled robots view a piece of artwork by artist Philip Colbert in the Lobsteropolis exhibition at the Saatchi Gallery in London, Thursday, Oct. 29, 2020. Lobsteropolis offers audiences the chance to view artworks in situ, or digitally via robots from the comfort of their own home. (AP Photo/Kirsty Wigglesworth)
Georgia Meacham attends a private view of Sophie Tea Art: Send More Nudes' Show in London. (Photo by Brett Cove / SOPA Images/Sipa USA)
EDITORIAL USE ONLY Kiera-Lorelle (left) , co-owner of organic food hall Cinnamon Leaf and Nicola Mendelsohn, Vice-President for EMEA at Facebook at the unveiling of a new shop front, created by renowned window designer Petra Storrs, as Facebook continues to support female-owned businesses across the UK, London.
Mollie (right), 7, and Emily, 9 in front of Halloween decorations on houses in Stormont, Belfast. (Photo by Liam McBurney/PA Images via Getty Images)
Rain falls in Nottingham ahead of the region being moved into Tier 3 coronavirus restrictions on Thursday. The very high level of restrictions includes a ban on social mixing both indoors and in private gardens, pubs and bars closing unless they can operate as a restaurant, and residents are advised against overnight stays in other parts of the UK and they should avoid travel where possible in and out of the area, unless it is for work, education or caring responsibilities.
Swans and geese float on the water of the Round Pond in Kensington Gardens in London, England, on October 26, 2020. (Photo by David Cliff/NurPhoto via Getty Images)
Tottenham's Son Heung-min celebrates after scoring his team's first goal during the English Premier League soccer match between Burnley and Tottenham Hotspur at Turf Moor stadium, Burnley, England, Monday, Oct. 26, 2020. (Lindsey Parnaby/Pool via AP)
Young ballet dancers perform during a protest calling for more funding for the performing arts in Parliament Square, London. (Photo by Stefan Rousseau/PA Images via Getty Images)
ST CYRUS, SCOTLAND - OCTOBER 26: A small bird perches on some broom at St Cyrus National Nature Reserve, on October 26, 2020 in St Cyrus, Scotland. (Photo by Ken Jack/Getty Images)
The Hero of a Hundred Fights, c.1800-10, reworked and exhibited 1847, at a press view for the forthcoming Turner's Modern World exhibition at the Tate Britain in London which is dedicated to JMW Turner and explores what it meant to be a modern artist during his lifetime. (Photo by Yui Mok/PA Images via Getty Images)
TETBURY, ENGLAND - OCTOBER 26: Camilla, Duchess of Cornwall during a visit to Westonbirt, The National Arboretum on October 26, 2020 in Tetbury, England. The Arboretum attracts over 550,000 visitors a year providing a beautiful landscape that people can visit to enjoy and learn about trees, as well as a vital resource for scientists to research more about these plants and understand how to protect them for the future. (Photo by Samir Hussein/WireImage)
Britain's Prime Minister Boris Johnson pulls down his face covering, worn due to COVID-19, to take a drink as he sits with hospital workers during his visit to Royal Berkshire NHS Hospital in Reading, west of London on October 26, 2020, to mark the publication of a new review into hospital food. (Photo by Jeremy Selwyn / POOL / AFP) (Photo by JEREMY SELWYN/POOL/AFP via Getty Images)
Staff at the Kitchen cafe in Bermondsey, London, are providing free school meals for children over the half term holidays. Local councils and businesses are continuing to pledge free food for children in need during this week's half term break after the Government defeated a Labour motion to extend free school meals provision in England over the holidays. (Photo by Stefan Rousseau/PA Images via Getty Images)
CARDIFF, WALES - OCTOBER 26: Kitchen products deemed 'non-essential' are seen covered up in a Tesco store on Western Avenue on October 26, 2020, in Cardiff, Wales. Pressure has mounted on the Welsh Government to reverse the decision to prohibit supermarkets from selling 'non-essential' items such as clothes and microwaves during the 'firebreak' lockdown period with a petition being submitted to the Senedd with over 65,000 signatures. (Photo by Matthew Horwood/Getty Images)
Lyn Friel, founder of the Crosskennan Lane Animal Sanctuary in Antrim, holds a kitten. (Photo by Liam McBurney/PA Images via Getty Images)
7-year old Wilf Waller takes a bag of food offered by volunteers in London Fields, east London, who are providing free school meals for children over the half term holidays. Local councils and businesses are continuing to pledge free food for children in need during this week's half term break after the Government defeated a Labour motion to extend free school meals provision in England over the holidays. (Photo by Yui Mok/PA Images via Getty Images)
Former paratrooper John Bream poses for a photograph before he attempts to set the record for highest jump without a parachute by jumping 40m from a helicopter off Hayling Island in Hampshire. (Photo by Andrew Matthews/PA Images via Getty Images)
A view of a deserted street on the first Sunday of Wales' national lockdown. (Photo by Andrew McCoy / SOPA Images/Sipa USA)
SOUTHAMPTON, ENGLAND - OCTOBER 26: The Nave Andromeda arrives to port on October 26, 2020 in Southampton, England. The 228-metre, Liberian-registered oil tanker was carrying oil from Nigeria to the port of Southampton when it was subjected to a suspect hijacking by stowaways off the coast of the Isle of Wight on the South Coast of the British Isles. The Ministry of Defence and Home Secretary authorised Armed Forces personnel to board the vessel in response to a police request and the incident was brought to a close in just nine minutes. (Photo by Finnbarr Webster/Getty Images)
Wolverhampton Wanderers' Conor Coady during the English Premier League soccer match between Wolves and Newcastle United at Molineux Stadium in Wolverhampton, England, Sunday, Oct. 25, 2020. (Nick Potts/Pool Photo via AP)
The P and O cruise ship Azura travels up Southampton Water into the port at Southampton, Hampshire. (Photo by Andrew Matthews/PA Images via Getty Images)
LONDON, ENGLAND - OCTOBER 25: Alex Lacazette of Arsenal before the Premier League match between Arsenal and Leicester City at Emirates Stadium on October 25, 2020 in London, England. Sporting stadiums around the UK remain under strict restrictions due to the Coronavirus Pandemic as Government social distancing laws prohibit fans inside venues resulting in games being played behind closed doors. (Photo by Stuart MacFarlane/Arsenal FC via Getty Images)
Liverpool's Mohamed Salah, rear in red, scores his side's second goal passing Midtjylland's goalkeeper Mikkel Andersen during the Champions League Group D soccer match between Liverpool and FC Midtjylland at Anfield stadium, in Liverpool, England, Tuesday, Oct. 27, 2020. (Peter Powell/Pool via AP)