Five police officers were injured at an unlicensed music event at the weekend.
Lincolnshire Police said it became aware of the illegal gathering near Holbeach St Marks at about 04:00 BST on Sunday.
It said about 200 people were at the event that finished at about 15:00. Current rules allow only six people to gather socially in one place.
Four people were arrested, two on suspicion of being violent and two for possession of drugs, the force said.
Det Ch Supt Jon McAdam said the force had monitored the event and engaged with some of those attending reminding them of the rules relating to gatherings.
He said five officers received minor injuries.
"Our officers do not come to work to be assaulted," he said.
"None of this is acceptable at any time, but during a national pandemic where we are seeing COVID-19 cases rising it is beyond words what people are actually thinking by organising and attending these events."
Mr McAdam said the force's response had been both "proportionate and pragmatic", and despite the music being turned off after officers arrived, it took several hours to clear the site whilst ensuring everyone's safety.
An investigation is now under way, he added, and officers have taken records of the vehicles which were at the scene.