Mid and West Wales Fire and Rescue Service says 11 of its crews are continuing to fight the fire at the National Library of Wales.
It is advising people "to stay away for safety."
This video - from viewer Edward Mulgrew - shows firefighters pumping water onto the library roof earlier.
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Patchy, light rain and drizzle will continue this afternoon. Brightening from the west later. A mild and dry night to come.
With the Welsh Government expecting major reductions to its spending budget, those working in education are concerned.
A 28-year-old man has been arrested on suspicion of murder after a 36-year-old man died from stab wounds.