Hundreds of tonnes of rubble have fallen on to a house after a wall at an 11th-century castle collapsed.
Emergency services including a hazardous area response team are at the scene in Lewes, East Sussex.
A pub has been evacuated as emergency responders seek to make the area safe.
Residents and shoppers are being asked to avoid the main centre of the historic town.
East Sussex Fire and Rescue Service confirmed crews were called to the scene at 12.22pm, along with search and rescue staff.
Incident commander Matt Lloyd said the wall had collapsed into an adjacent garden and house.
Speaking to the PA news agency, he said the rubble was estimated to have had “hundreds of tonnes of impact”.
“I would say this is a protracted incident.”
The wall was reportedly undergoing work to remove ivy in recent weeks.
A police dog team has been brought in to help assess the integrity of the structure.
The large emergency response is made up teams from three counties, including specialist search and rescue teams trained to work in earthquake scenarios.
A fire service spokesman said: “Residents are being asked to avoid the area, as there are expected to be a high number of emergency appliances involved in this multi-agency search and rescue operation.
“Firefighters are working alongside other emergency services to check the area to ensure that no one needs rescuing.”