The largest hospital in Wales has issued a business continuity incident - its highest level of alert.
Cardiff and Vale University health board said it was experiencing "significant and sustained adverse demand on services".
The board said in a statement the demand was greatest within the emergency unit, situated at the University Hospital of Wales.
The hospital, located in the Heath area of Cardiff, is the largest in Wales.
A spokesperson for the board said the situation was exacerbated by "seasonal pressures and limited bed availability" as people were staying in hospital for longer.
The public is being urged to only attend the emergency unit in the capital in cases of a "genuine life-threatening emergency".
Currently, there is "no disruption" to pre-scheduled appointments which "continue to operate as usual", the health board confirmed.
"Cardiff and Vale University Health Board would like to take this opportunity to thank the public for their patience and support in what is an exceptionally difficult time for colleagues," the spokesperson added.
The Welsh Conservatives' shadow health minister, Russell George, said the situation at the hospital "eptimoises the crisis at the heart of our Labour-run health service".