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
It'll be a mainly dry but chilly night for most of us with a risk of patchy frost, fog and low cloud later.
Ched Evan's fiancé Natasha Massey talks for the first time about why she has stood by him for the last five years.
Leanne Wood has said she accepts the referendum result but that it has left Wales with a weak position for negotiation.