Under heavy skies that broke open more than once, Federation Square swelled with supporters seeking clemency for Myuran Sukumaran and Andrew Chan, the two Australian prisoners facing the firing squad in Indonesia.

The prevailing mood among the hundreds who gathered for the vigil on Wednesday night was hope rather than resignation.

Speakers told the crowd that a delay in the pair's transfer to their final place of execution had bought them precious time and that there was still a chance their lives would be saved.

But as bells rang out from St Paul's Cathedral and drifted across the city, they also toll for Sukumaran and Chan in Bali.

Artist and friend Matthew Sleeth, who organised the vigil, spoke about how the pair had changed their lives for the better in prison and that they would make the community a better place if given a second chance.

He said he never thought the day would come when Sukumaran and Chan would be executed.

"If these executions go ahead we will all be diminished, both Australians and Indonesians," he said.

Musicians Clare Bowditch, Bob Evans and Missy Higgins showed their support by performing for the vigil. Matthew Goldberg, from Mercy, said there had been nearly 200,000 signatures on a petition for clemency.

Family Drug Support founder Tony Trimingham, whose son died of a heroin overdose, said the worst pain was yet to come for the families of Sukumaran and Chan, should the convicted drug smugglers be put in front of the firing squad.

"They'll be dreaming of their sons being alive and waking up to the nightmare of their early deaths," he said.

"They will be tortured on birthdays and anniversaries. They will miss whatever the future would have held."

In a letter read by MC Eddie Perfect, Chan asked those who had attended to think about the meaning of the word mercy.

"This campaign is more than just about myself or Myu," he wrote.

"It represents a second chance to forgive, it represents kindness and help for those in a helpless situation."