Hundreds of Australians protested on Saturday against the treatment of 600 refugees who are refusing to leave a former prison on Manus Island for fear of being attacked by locals.

In a crackdown against "boat people" arriving in the country, Australia set up prison camps in the Pacific islands of Manus and Nauru in Papua New Guinea (PNG), nearly five years ago, to process asylum seekers.

The United Nations and human rights defenders have condemned the treatment of asylum seekers amid allegations of rights abuses, and demanded the camps be shut down.

The Manus Island facility was decommissioned on Tuesday, but the 600 men there barricaded themselves in without food and water in defiance of Australian and PNG authorities. They are asking other countries to take them in.

"We are pleading the international community to rescue us," said asylum seeker Abdul Azziz Adam in a video message from the camp in Papua New Guinea. "What is going on on Manus Island at the moment is a humanitarian crisis and no one will help us."

The men have resorted to drinking rain water and have dug their own wells in order to survive. The UN has called the crisis an "unfolding humanitarian emergency".

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Australia's government says the 600 men have been offered alternative accommodation elsewhere on Manus Island, but the refugees say it is not safe from locals who want to do them harm. Many asylum seekers have been attacked in the past by Papuans who do not want them on the island.

New Zealand has offered to take in 150 asylum seekers, and its Prime Minister Jacinda Ardern will arrive in Sydney on Sunday to discuss the situation with her counterpart, Malcolm Turnbull. So far, Australia has rejected the offer.

"We are begging the New Zealand government to rescue us because New Zealand is the only country today, they have been fighting again and again and again, they want to get us out of detention centre," Adam said in the video message.

Al Jazeera's Andrew Thomas, who attended a protest rally in Sydney on Saturday, said the central message among the crowd was for the government to "bring them here".

"I think the situation on Manus Island is completely and utterly deplorable," a young, male demonstrator told Al Jazeera. "I think it's really important that the government takes a stand … so we can bring these men who are scared and suffering to Australia."

Hundreds of protesters also converged in Australia's second-largest city, Melbourne, on Saturday.