Approximately half of the roof over Bethel Chapel on New Road, formerly the A483 in Newtown, caved in overnight, sparking fears that more of the building could fall onto the road.
Heavy rain lashed the area in the early hours of Monday.
Emergency services went to the scene after 999 calls and police closed the road.
It is thought the road will remain closed for much of the day.
The church has been closed to the public for some time, and there had been concern about the structure of the building.