ARIA award winners Clare Bowditch and the John Butler Trio headlined the free show, organised by lobby group The Wilderness Society.

The group opposes a plan backed by the West Australian government to build a $53 billion gas plant in James Price Point, located 55 kilometres from Broome.

The audience, mostly made up of young people, packed the lower and upper sections of the square, with the atmosphere livening up when artist John Butler took to the stage after 7.30pm.

While some wore T-shirts with slogans and held political signs; a majority were music fans and curious onlookers.

Former federal Greens leader Bob Brown addressed the crowd, while singer Missy Higgins sent a video message from the United States.

Dr Brown said the large crowd was "a powerful message from Melbourne to Canberra".

Former Broome local James Psembi, 21, said he was pleased to see "Broome-related news trickle down to Melbourne".

Mount Dandenong resident Phoebe Kramer, 18, described the mood as "friendly".

"It's a good cause, although a lot of us are learning about why we're here," she said.