Police, fire and ambulance have been scrambled to the scene, with a helicopter hovering near the top of Dixon's Chimney in Carlisle, Cumbria.
Footage from the scene appears to show a figure, upside down with his legs in the air, at the top of the chimney.
Roads around the structure, built in 1836 and a prominent local landmark, have been closed.
Local reports said shouts and wailing could be heard coming from the chimney in the early hours of this morning before police arrived on the scene.
In a statement Cumbria Police said: "Police are continuing to deal with an incident at Dixon's Chimney, Carlisle.
"Officers became aware of a man trapped at height in the area following a call to police.
"Officers are supporting Cumbria Fire and Rescue Service and other partner agencies to attempt to free the man safely.
"The road is currently closed at Stanhope Road, Charlotte Street and Shaddongate where it meets Bridge Street (A595).
"The public are asked to avoid the area."