Llanelli Library was evacuated on Monday afternoon due to a fire in the roof of the building.
Emergency responders were quickly on scene just after 2pm and roads and pedestrian walkways around the area were cordoned off.
Staff raised the alarm after smelling smoke, and quickly evacuated the building.
Mid and West Wales Fire and Rescue Service used thermal imaging cameras and hose reel jets on the first floor and roof area, and left the scene at around 5.30pm.
The library was re-opened to visitors as normal on Tuesday morning.
Investigations are now underway to determine the cause of the fire and the extent of any damage, although the main parts of the building are entirely unaffected.
The Carmarthenshire County Council library, in Vaughan Street Llanelli, was recently named one of the busiest in the country, listed as the third in terms of lending figures for books and other items in the UK.
Cllr Peter Hughes Griffiths, Executive Board Member for Culture, Sport and Tourism, said: “We were grateful of the quick response of emergency services.
“The quick and calm response by library staff meant that everyone was safely evacuated within minutes of the alarm being raised.
“We’d also like to thank our neighbours at Llanelly House who accommodated staff during the afternoon.
“We will now be investigating the cause of the fire and any damage, although we are fortunate that public areas of the building were unaffected and have re-opened as normal.”
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