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
Ahead of tonight's TV debate, Welsh Labour has hit out at Plaid Cymru describing it as a 'minority party.'
Extremists are using rural parts of west Wales to train and radicalise people, that's according to an officer from an anti-terrorism unit.
To mark World Autism Awareness Day, Welsh care group Orbis are launching a new campaign to change the way Autism is perceived.