Protesters set fire to Old Parliament House in Canberra

Police have confirmed a fire at Old Parliament House this morning was started by protesters.

The fire has caused significant damage to the front facade of the building, with protesters outside heard chanting "let it burn".

The protest was initially peaceful but later escalated, with police using pepper spray.

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"It then got a little bit out of hand and then when ACT Police went to respond it was greatly exacerbated," Commander Linda Champion from ACT Police said.

No arrests have been made, however an investigation is underway.

It is believed the protesters are part of an anti-vaccination group, however Commander Champion did not provide any details about them, pointing to the ongoing investigation.

She added there had been several protests outside the front of the building in the last few weeks.

A statement from the Aboriginal Tent Embassy in Canberra condemned the actions of today's protesters.

"Although we support the concept of non-violent direct action, we do not condone the destruction of public and private property," the statement read.

"The actions of such protestors conducting a 'Smoking Ceremony', was done so without the knowledge, consent or mandate of the Embassy Council and Traditional Owners responsible for the regulation of the Aboriginal Tent Embassy."

ACT Fire and Rescue superintendent Sam Evans confirmed police and firefighters were called to the historic building around 11.45, after receiving multiple Triple Zero calls.

The building was evacuated as firefighters tended to the blaze.

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The fire was put out within 15 minutes, but extensive damage to the front of the building and portico had already been done.

The museum will remain closed for some time, as investigations continue and the extent of the damage is assessed.

Mr Evans said smoke from the fire and water from the building's sprinkler system had caused damage, but the fire did not spread.

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"It's been confined to the front facade of the building," he said.

Andrew Harper from the Australian Museum said Old Parliament House would be closed temporarily.

"Obviously we're devastated by the damage to the building," Mr Harper said.

He added the destroyed front doors were an iconic part of the building, part of the original 1927 structure.

It's unclear whether the collection inside has been damaged.

Melbourne chef Adrian Richardson, who witnessed the blaze, has told 3AW he saw the front balcony above the front steps burst into flames.

He said protesters near the site were chanting "let it burn".

"There's a lot of police, there's a lot of police with flags all waiving and screaming, a lot of fire trucks," he told Shane McInnes.