Placards of "bring them home" and "different colour, same rights" were held aloft during a song calling for the Morrison Government to allow asylum seekers in the country.
The crowd chanted: "Bring them here".
Friday marked six years since the Rudd Labor government reintroduced offshore detention for asylum seekers who arrived by boat.
Hundreds have also joined rallies in Sydney, Melbourne, Brisbane, Adelaide, Hobart and Darwin on Saturday afternoon after protests kicked off in other parts of the country earlier in the day.
Comedian Dan Ilic spoke to the crowd at Sydney's Town Hall, saying the potential of asylum seekers was being wasted.
"What struck me by meeting the refugees on Manus Island was just how funny, charming and clever they were, and how throughout this entire debate, throughout this whole process of six years we have stripped them of their humanity," he said.
Refugee Shaminda Kanapathi, who has been on Manus Island since 2013, addressed supporters via video link in Brisbane.
The 28-year-old says men detained on the island have no control over what happens to them.
"Even the most intimate decisions are made for us," Mr Kanapathi told AAP on Friday ahead of the rally
"Our lives have stopped. And every minute passes, as we have lost six years already."
Author Benjamin Law has also thrown his support behind the cause.
"For many decent-minded Australians, it's obvious ... Australia can do better, and policy-makers must work harder towards a humane solution that cuts through the falsehood that the only two choices are permanent imprisonment or deaths at sea," Law said.