ACT Emergency Services were called to the scene to find the front doors alight just before noon on Thursday.
The building was evacuated as a precaution and crews extinguished the blaze.Â
Social media footage shows police dragging protesters away from the front steps of the building. It also shows a large fire burning on the doors.
The building was also set on fire by protesters on December 21.
ACT Policing said it was the continuation of a series of recent protests.
"Old Parliament House was evacuated earlier today 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."
Deputy Prime Minister Barnaby Joyce said it was "an absolute disgrace" if the fire was deliberately lit.
"This is the parliament that for so long underpinned the freedoms we have as a democracy and made the decisions that formed us as a nation, fighting for the increase of rights and corrections to our colonial past," he said.
"If someone is trying to make a statement then it is a very bad one that will be received with overwhelming disgust."
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".
"I had the great honour of working for Tom Uren in that building. It's used by school groups, it's used for educational purposes as well as for functions," he said.
"It's an important building and I just hope that there is only minimal damage."
The building is now home to the Museum of Australian Democracy.
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