This video - from viewer Mark Bloomfield - shows people gathering outside, as fire crews tackle the blaze on the National Library of Wales' roof.
It is thought that nobody will be allowed back inside the library until tomorrow.
Workers were all evacuated after the fire broke out.
More top news
There are calls for dog owners to do more to control their animals after a spate of attacks on dogs in Port Talbot.
Cloudy with rain continuing, mainly affecting western-facing hills.
The last surviving members of the north Wales branch of the Normany Veterans Association held their last formal meeting today.