Federal Greens MP Adam Bandt has labelled Immigration Minister Peter Dutton a "terrorist" at a rally in Melbourne protesting against the treatment of men who remain on Manus Island.
More than a thousand people gathered at the State Library to show their support for 600 refugees and asylum seekers who remain at the Manus Island detention centre in Papua New Guinea, despite its official closure on Tuesday.
The men say they do not feel safe in the replacement accommodation provided for them in the community.
Mr Bandt told the crowd the men had been "thrown in prison" by the Australian Government.
"These people have committed no crime other than to do what every single one of us would do if we thought our lives, or our family's lives, were at risk," he said.
Mr Bandt likened the Immigration Minister to a "terrorist" for "threatening people's lives".
"If the definition of terror is to use violence and threaten people's lives for political purposes, then Peter Dutton is a terrorist," he said.
"To look at the face of Peter Dutton is to stare into the eyes of someone who is prepared to kill people for political gain, and it's time he was held to account for this crime against humanity."
He also criticised Labor for reopening the offshore processing centre while in government, and urged them to join the Greens in their push to bring the men to Australia.
The crowd later moved down to Flinders Street Station and Federation Square, where they staged a mass sit-in.
Trams and traffic were brought to a standstill, while access to the train station was disrupted.
Hundreds also gathered at a similar rally in Sydney.
The UN has said the Federal Government should provide immediate protection, food, water and other basic services to men, calling in an "unfolding humanitarian crisis".
The UN said there were serious concerns about the safety and wellbeing of the men, and both Australia and PNG were responsible under international human rights law to protect them.
Mr Dutton's office said it would not comment on the UN's statement but has previously said there would be no change in government policy to allow the men entry to mainland Australia.
Opposition Leader Bill Shorten, who has urged the Federal Government to consider New Zealand's offer to help resettle 150 of the men, said the UN "had a point".
"We don't want to see the people smugglers back in business, but I think there is something going on at Manus which is deeply disturbing to the Australian people," he said.
"Where you have got 600 people without food and water for days, the Government needs to take an active interest in their welfare."
Meanwhile, Papua New Guinean soldiers have been instructed not to allow anyone to bring food to the refugees inside the detention centre.
Food, power and water supplies to the centre were cut when the site was officially closed.
A soldier at the gate to the Lombrum naval base, in which the centre is located, says a church group with food for the refugees was turned away on Thursday.
On Friday, soldiers prevented a locals from landing a boat carrying food near the centre.
The 600 men inside say conditions are worsening but they remain determined to stay.