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.
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A very unsettled day with frequent outbreaks of persistent, occasionally heavy rain, especially in the south and over hills.
Protesters are set to march through Cardiff city centre today to campaign against local budget cuts to the arts.
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