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
A collection of thousands of Welsh newspapers have been moved to a new storage facility, along with millions of British editions.
ITV Wales' current affairs programme made for S4C have been presented with an award for coverage of the transgender community in Wales.
A dry and bright afternoon ahead becoming increasingly sunny through the day, but temperatures staying low. Max temp 5 deg C