Lobby group Get Up has organised a series of Snap Action rallies across the country as a landmark ruling is handed down on whether the federal government has power under the constitution to detain people in Australian-funded detention centres at Nauru and Manus Island.

Canberra's Refugee Action Committee said Canberra residents were among those gathering about 11am to protest the deportation of 267 asylum seekers, including 37 babies born in Australia.

All were brought back to Australia from offshore centres in Nauru or Manus Island, mostly for medical treatment but may be returned within a matter of days should the court find offshore detention is lawful when it hands down its ruling from 10.15am.

Dr John Minns of the Canberra's Refugee Action Committee said each of the asylum seekers was "terrified" of being sent back to Nauru.

"The appalling conditions in Australia's offshore detention centres are well-known. Countless health care workers and other whistle-blowers who have been in these centres have testified to the physical, mental and sexual abuse of asylum seekers they have witnessed," he said.

Another rally in support of refugees will take place on the corner of Barry Drive and Northbourne Avenue at 5.30pm on Thursday. Greens Senator Sarah Hanson-Young is expected to address that rally.