North Wales Police have been called after more demolition work at the former Mines Rescue Station in Wrexham.
On Wednesday demolition on the 1913 building was stopped by the local council, which said a safety certificate was needed.
Heritage body Cadw then listed the building, giving it protection.
But police were contacted on Friday after a demolition vehicle was seen entering the premises again, and the area has now been taped off.
Local councillor Phil Wynn said the most historically important part of the building had been destroyed.
Mr Wynn said he notifed the council's building control and conservation officers as soon as he heard demolition work was under way.
He said: "It's quite obvious that the training gallery and drill hall are lost and they were integral parts of the mining heritage of the building.
"We've been trying to bring the importance of the building to Cadw's attention for the last two years from a heritage point of view.
"Having seen the damage that's done, for safety reasons it's going to have to be demolished to the rear."
North Wales Police confirmed officers were called just before midday after reports of damage to a listed building.
Work was stopped when police arrived and officers interviewed the contractors.
Police said the matter was now being dealt with by Wrexham council's planning office.
Attempts were made by the BBC earlier in the week to contact the building owner, Neville Dickens, but he could not be reached.
Cadw listed the building for its "special historic interest" after campaigners called for the protection.
A spokesperson for the heritage body said: "The former Mines Rescue Centre in Wrexham was recently recommended for listing by Cadw subject to a four week consultation until 3 September.
"On Wednesday bulldozers started removing parts of the rear of the building.
"The local authority ordered a stop of this work but because of the immediate threat to this building Cadw decided to list it with immediate effect.
"On the evidence currently available to Cadw the building has been listed for its special historic interest as a mines rescue building, purpose built to serve the mines within the North Wales coalfields and important for retaining significant original form and layout, in particular the training gallery.
"It is a rare example in Wales and displays good architectural detailing and construction."
Planning permission to demolish the building was granted two years ago.
Those who trained at the station helped in the mission to rescue miners from the 1934 Gresford disaster, when an explosion claimed the lives of 266 people.
The centre closed in the 1980s and was handed to the fire service, but it recently passed into the ownership of businessman and former Wrexham FC chairman Neville Dickens.