More than 1,000 people have rallied in Canberra's CBD in support of asylum seekers who are being held in detention centres on Manus Island and Nauru.

The pro-refugee rally saw scores of religious groups, education institutions, unions and community organisations converge on Garema Place.

The rally was part of a national day of action to condemn the Federal Government's policies and treatment of refugees.

Whistleblower Tobias Gunn told the crowd of the horrors he saw while working with Save the Children Australia at the Nauru detention camp.

He said he was haunted by the physical and emotional abuse he witnessed, including the bashing of a toddler and the sexual advances made by guards to young girls.

"Why did we as Australians put vulnerable people in a system of institutionalised abuse called indefinite detention?" he said.

"A place where every month two children are sexually abused, every week two children lose hope and self harm, a place where every second day a child is assaulted.

"A place where almost half of the physical and sexual abuse is at the hands of Commonwealth employees."

The rally called for the Government and opposition to reconsider their refugee polices.

Australian Muslim Voice ACT's president Diana Abdul Rahman and Tobias Gunn in front of the crowds. ( ABC News: Alkira Reinfrank )

"We need to demand from both parties, Labor and Liberal, to stop using this topic for pathetic point scoring, to do what they have lost sight of, their job, to lead," Mr Gunn said.

"Let's not turn a blind eye. History will look back on this and how we acted. Our very own children will ask us, 'how did you let this happen?'."

Mr Gunn said the plans to process all asylum seekers on Nauru were a "very small step in the right direction".

"However let me tell you from experience, it is little better," he said.

"These people still have no future, it will be a five-year visa until they are moved onto a third unknown country."

Along with Mr Gunn, speakers including Australian Muslim Voice ACT's president Diana Abdul Rahman and Unions ACT president Alex White addressed the vocal crowd.