Police discovered the party at a unit on Phoenix Business Park in Stafford Street, Walsall, in the early hours of September 14.
Officers initially had the shutter slammed shut in their faces after going to investigate following people and cars turning up.
Police started taking details of the cars parked up when one of the gates – leading to the units – were locked, trapping people inside.
Some party-goers jumped over a wall to escape before West Midlands Police told the person to open up – which led to scores of people leaving.
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Music equipment, food and drinks were found inside the unit – with the man renting it admitting to owning the sound equipment and laptop.
He has since been slapped with the £10,000 fine on October 10 – days before Walsall had more virus measures enforced – over hosting the rave.
Chief Superintendent Andy Parsons, from Walsall police, said: "We’ve been warning anyone planning to organise or attend illegal raves not to do so for many months.
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"This event was particularly dangerous, not only due to the pandemic, where large gatherings are banned, to help stop the spread of the virus, but it was also a fire hazard and the fact the shutters and gates were locked for periods preventing people from getting in or out was reckless.
"Fortunately officers spotted it and were able to close it down before something serious happened.
"After extensive enquires, we’ve now been able to use the new legislation to issue the fine which I hope will deter others from holding similar events.
"I’d warn anyone considering attempting to arrange or attend these events, which are not only dangerous, but also illegal, not to.
"If you’ve been invited to one or are aware of any planned to take place please let us know.
"Your help is invaluable, as the key to stopping these events is early intelligence from you."
Anyone who suspects a rave or illegal gathering is being organised should contact West Midlands Police on their website or by calling 101.