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At it's height, almost 100 firefighters and officers were trying to put out the fire at the Cuming Museum in Walworth.
Emergency services were first called to the scene shortly after 12.30pm on Monday 25th March 2013.
A fire bridgade spokesperson said:
"There are currently no indications of how the fire started and no injuries have been reported."
Later a fire service spokesman said the whole of the roof of the museum and library was on fire.
Station Manager Peter Critchell, at the scene, said:
"Flames are coming through the roof and there is thick grey smoke billowing up. Firefighters are still working hard at the scene to try and bring the blaze under control.
"We now have water jets attacking the fire from the four corners of the building. We evacuated 30 people from inside the museum and firefighters will be making efforts to salvage as many of the artefacts as possible."
Thick plumes of smoke and flames could be seen coming from the building's roof.
Onlookers were kept back by a cordon and took pictures with phone cameras as firefighters battled the flames using aerial platforms.
Peter Critchell, a spokesman for London Fire Brigade at the scene, said 30 people had been evacuated from the property using normal procedures.
"We will be here for some considerable time yet - we are trying to make sure the fire is well and truly under control and then after that there will be a lot of salvage work, cutting away and damping down and an investigation.
"At the moment the cause of the fire is a matter for investigation and we have no information on how it started."
Richard Rees, a 62-year-old pensioner who works in the community, said:
"When I got here the flames were going. The fire services arrived very quickly and did a fantastic job.
"All staff got out very quick and professional, they got out safe.
"They really didn't know what was going on until they got out and looked up because it was an upper fire.
"It's a listed building, it was one of the very few treasured buildings in the community.
"It took a long time to get it listed and it is a very sad day for us.
"Parts of it are Grade I listed and parts Grade II.
"It is Victorian and was the old town hall before the big boroughs were set up.
"It is one of three in the borough which the council has been trying to sell off."
The Leader of the Council, Councillor Peter John, said:
"Our thoughts are with everyone affected and we will offer what practical assistance we can"
"It is too early to say what the cause of the fire is and how long services from that building will be suspended for."