Thousands of people have turned out in Melbourne, Sydney and Brisbane to attend #LetThemStay rallies.
Crowds also gathered at rallies in Darwin and Canberra.
The protesters are calling for the federal government to allow 267 asylum seekers, including 37 babies, who came to Australia for medical treatment, to be allowed to stay.
The federal government has said it was free to return the asylum seekers to Nauru after the High Court ruled the government was legally allowed to maintain regional processing centres.
The crowds held signs calling on the government to allow the people to remain in Australia and carried photographs of some of the 37 babies who were among the group at risk of being sent back to Nauru.
Greens leader Richard Di Natale attended the Sydney rally.
Some on social media claimed a crowd of 6000 had turned out in Melbourne, but SBS News has not been able to confirm this figure.
In the last two days the Premiers of NSW, Victoria, Queensland and South Australia, and the ACT chief minister, who declared their willingness to provide homes for the asylum seekers.
