CCTV images of an attempted robbery on Hemswell Antique Centre have been released.
Two female staff were opening up the centre on the Hemswell Cliff industrial estate, near Gainsborough, at 9.45am on Wednesday when they were approached by three men who had been waiting inside.
The women fought off the men and then managed to activate a hidden alarm.
The intruders, who were all wearing Halloween masks, then fled via a window.
It is believed they left the scene in a blue 4x4 vehicle travelling across the field at the back of the unit.
The men are believed to be white and were all wearing dark clothing.
Anyone who thinks they can help should contact Detective Constable Tina Kennedy of Gainsborough CID via the non-emergency 101 number, quoting incident number 111, or Crimestoppers anonymously on 0800 555111.