ACT Emergency Services were called to the scene to find the front doors alight just before midday on Thursday.
The building was evacuated as a precaution and crews extinguished the blaze.Â
Social media footage shows police dragging away protesters from the front steps of the building. It also shows a large fire burning at the doors.
A fire was previously started by protesters near the building on December 21.
ACT Policing said Thursday's blaze was the continuation of a series of recent protests.
"Old Parliament House was evacuated ... after protesters started a fire at the front doors of the building - this was quickly extinguished by ACT Fire and Rescue," a spokesperson said.
"There has been ongoing protest activity at the front of Old Parliament House throughout the past fortnight.
"A police investigation into the cause of the fire has commenced. Old Parliament House will remain closed until further notice."
A group of COVID-19 anti-vaccine campaigners broadcast their protest and parts of the fire on social media.
One protester told Sky News the group was there "for positive reasons" but added they "had to burn that door down just to get the message across".
Prime Minister Scott Morrison said he was "disgusted and appalled" by news of the fire.
"This is not justified. This is not how Australia works," he said.
"It is appalling and I think it is disgraceful, and I think that the authorities should act swiftly in accordance with the law and people should face consequences of their actions.
"We have a rule of law in this country and people should obey it."
Former deputy prime minister Michael McCormack said repercussions should be "swift and severe" if it was arson.
"An outrageous attack on our democracy, our history and sovereignty," he said.
"This penchant for tearing down our past serves no purpose."
Opposition Leader Anthony Albanese said it was a "very historic building".
"It's an important building and I just hope that there is only minimal damage."
The building, completed in the 1920s, is home to the Museum of Australian Democracy and was added to Australia's National Heritage List in 2006.
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