Police shut down a house party attended by more than 300 people at a hired venue in Midlothian.
The “blatant disregard” for coronavirus restrictions took place at Kirkhill House near Gorebridge on Saturday night.
Officers arrived to break up the rave just after midnight and charges against the organisers are being considered.
A 29-year-old man was issued with a fine at the scene.
Current coronavirus-fighting restrictions limit the number of people gathering indoors to eight from three different households.
Deputy chief constable Malcolm Graham said “Those attending this organised event showed a blatant disregard for the regulations in place to help save lives and prevent the spread of coronavirus.
“Further investigations are being carried out with a view to what other charges may be brought, including culpable and reckless conduct.
“Anyone attempting to hold such an event, or any party in clear breach of the law, should be aware they will be met with a robust response from Police Scotland.”
Local councillor Stephen Curran said the “selfish” party goers had put their own and others’ health at risk.
He said: “To call them selfish and irresponsible is an understatement. We realise it can be challenging following the guidance but if the majority of us can do it then so can they.
“We are working hard to avoid lockdowns like those happening in and around Glasgow but we need everyone to help keep Midlothian safe. So, please follow the guidance. It will save lives.”
Police Scotland last weekend responded to reports of around 300 house parties across the country, dispersed those attending and issued six fixed penalty notices.
Five of those fines were issued after police attended a house party in Kirkintilloch, East Dunbartonshire, around 8pm on Saturday, August 30.
Two men were also arrested in connection with a disturbance at the property.