A 33-year-old man was hit in the face, tied up and beaten by five men who attacked him when he opened the door of a disused Brighton church.
Sussex Police said the victim, from Albania, was confronted by the gang at the door of St Augustine's Church in Stanford Avenue.
The attack happened at 21:30 GMT on Monday at the church, which is being converted into a community centre.
The gang demanded money and gold but made off with electrical items.
They are described as three British and two eastern European men.
"Their victim suffered some really nasty injuries and we are keen to speak to anyone who saw or heard anything suspicious," said Det Sgt Cheryl Lewendon.
"While this would appear to be an isolated incident, patrols have been stepped up in the area to provide reassurance to residents and visitors."