Firefighters were today tackling a blaze at the College of Arms, fire chiefs said.
Parts of the third and fourth floor of the six-storey office building on Queen Victoria Street in Blackfriars were alight, a London Fire Brigade spokesman said.
Eight fire engines and about 40 firefighters were tackling the blaze after they were called shortly before 11am.
The College of Arms is the official repository of the coats of arms and pedigrees of English, Welsh, Northern Irish and Commonwealth families and their descendants.
Its records also include official copies of the records of Ulster King of Arms, the originals of which remain in Dublin.
The fire service spokesman said: "Eight fire engines and around 40 firefighters currently are tackling a blaze at a six storey office building on Queen Victoria Street in the City.
"Parts of the third and fourth floor are currently alight.
"Fire crews from Dowgate, Islington, Clerkenwell, Soho and Euston are attending the incident."
The current college building, which is just south of St Paul's Cathedral, dates from the 1670s and replaced Derby Place, which burnt down in the Great Fire of London in 1666.
It is one of the few remaining government heraldic authorities in Europe and is overseen by the Earl Marshal, a hereditary office held by the Duke of Norfolk.
A spokeswoman for the current Duke of Norfolk, Edward Fitzalan-Howard, said he was unavailable to comment at the moment.
Later, a London Fire Brigade spokesman said 35 people were evacuated from the College of Arms, and a further 100 people were moved out of adjacent buildings.
The London Fire Brigade spokesman said the blaze was under control by 2.30pm.
He said there had been a "small amount" of damage to the fourth floor and to a mezzanine roof level.
"Firefighters were also involved in salvaging the work and different artefacts," he said.
The cause of the fire was under investigation, he said.