Crowds gathered on Sunday night to "commemorate" Anzac Hall, which will be demolished under a controversial redevelopment.
"The vigil itself will be a really lovely way to mark the 20 years of memories that the public and veterans and veterans families have had in the existing Anzac Hall," Australian Institute of Architects ACT President Shannon Battinson said.
The Australian War Memorial's $500 million expansion was given the green light by the National Capital Authority, clearing the way for the 20-year-old Anzac Hall to be demolished for a new gallery space.
About 140 trees are slated for removal as part of the project's early works.
About 100 people attended the sombre, candlelit rally calling for the War Memorial to "press pause and listen".
Ms Battinson supported expanding exhibition space but said it should be done in a way that maintained the War Memorial building as a prominent feature on the site.
"Come back to the architects with a brief that's actually befitting of that purpose and come up with a more appropriate solution," she said.
The project's approval followed a public consultation period in which the vast majority of submissions slammed the proposal, noting the heritage value of the hall and removal of native trees.
"This is not how democracy works," Ms Battinson said.
"At every opportunity where the public have been given any form of chance to speak, overwhelmingly, that voice has been against the project."
Memorial director Matt Anderson earlier told The Canberra Times the planned demolition and replacement of Anzac Hall would grow the institution's heritage value by creating more space to tell Australia's military history.
"If we're doubling the space available to tell the stories, we're doubling the heritage value of that building, and of the memorial's power to tell the story of continuing service and sacrifice," he said in April.
Ms Battinson was concerned the institution would become a museum to machinery, rather then about remembering the cost of war.
Stewart Mitchell was the former head of buildings and services at the War Memorial, he said the redevelopment didn't understand the "commemorative power" of the existing site.
"It's simply tragic to know that excellent concepts for development exist which provide the additional space required without destroying the site's national and Commonwealth heritage listed attributes; and at considerably less cost," he said.
Resident Jane Aitken said it "broke her heart" to see the hall designed to "blend in" go, alongside scores of decades-old trees.
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