An unlicensed New Year's Eve rave in Kettering was attended by up to 1,000 people, police say.
Police were called to reports of the music event on the Telford Way industrial estate overnight.
Officers attended the unoccupied warehouse and have been monitoring the event throughout the early hours of New Year’s Day.
The force has received a small number of complaints from nearby residents about the incident.
Supt Dennis Murray, from Northamptonshire Police, said: “This is a large-scale event which we became aware of after hundreds of people were already on site.
“The location is an empty warehouse on an industrial estate and due to the number of people at the location a decision has been made to let the event run its course.
"Organisers have told us they expect to have left the site by midday and the number of people at the event is expected to reduce as the morning progresses.
“I appreciate this has had an impact on people living near to the warehouse, and understand their frustration that we have not been able to close the event down.
"However, we have to consider other incidents taking place across the county and the resources available to us before taking action, and sadly New Year’s Eve is by far the busiest night in policing,
“We will continue to monitor the situation and will investigate any criminal offences reported to us as a result of the event.”