SYDNEY   -  Buildings were razed as hundreds of asylum-seekers escaped detention during riots at an Australian refugee facility on Nauru, witnesses said Saturday, following the launch of a hardline immigration crackdown.

Australia announced on Friday that boatpeople will no longer be resettled in the country and all future unauthorised arrivals will be sent to poverty-stricken Papua New Guinea for permanent resettlement. The riots on Friday night saw detainees take control of the immigration processing centre on the remote Pacific atoll of Nauru and arm themselves with makeshift weapons.

Nearly half of the facility's 545 asylum-seekers escaped and a number of buildings were set alight, according to local photographer Clint Deidenang.

"Today was history. The biggest riot ever to be staged on Nauru soil. The most violence I've seen. Amazing support from local(s) to the police," said Deidenang, estimating that 95 percent of the centre's buildings had been razed

"I can see workers in orange shirt(s) going through the war zone-like wreckage of burned and twisted tin houses. Destructive site," he wrote on Twitter.

The asylum-seekers abandoned their four-hour protest after a huge group of locals descended on the centre armed with pipes and machetes to help authorities contain the violence in response to a government call for assistance, he added.

David Adeang, acting president of the Micronesian nation with a population of just 9,400, issued an emergency decree granting private security forces and others police powers to respond to the riots.

Special laws were also passed overnight to allow detainees to be held for seven days for questioning without charge.

As of midnight on Friday Australia's immigration department said all the asylum-seekers had been accounted for and order had been restored. An immigration spokeswoman said that most of the major buildings including the accommodation blocks, medical centre, dining hall and offices had been destroyed during the protest, which involved about 150 detainees.

Four detainees had been hospitalised with minor injuries and no staff were hurt, she added.

Tents and marquees will be erected at another site as contingency housing, with initial estimates putting the cost of the damage at Aus$60 million

"I can confirm that there are currently 129 transferees at the police watch-house in Nauru," the spokeswoman told AFP.

"The ringleaders apprehended at the site will be questioned by police about allegations of property damage, destruction of property and riotous behaviour."

If convicted, she said those accused could be jailed for up to seven years and Nauru had vowed to prosecute them to the "full extent of the law".

Immigration Minister Tony Burke condemned the violence and warned that he could intervene to refuse people asylum on character grounds following the incident.

The Nauru centre is part of Canberra's punitive Pacific detention policy for asylum-seekers who pay people-smugglers for passage to Australia, with a second facility on PNG's Manus Island.

The programme was ramped up Friday with an announcement that Australia would banish all future asylum-seekers arriving by boat to PNG permanently, regardless of whether they are ultimately found to deserve a visa.

Australian Prime Minister Kevin Rudd hopes the hardline plan will boost his fortunes in an election year by stemming the flow of boat arrivals, which have exceeded 15,000 so far this year.

The first group of passengers to be affected by the new arrangement was intercepted off northern Australia in the early hours of Saturday morning.

Burke said the boatload of 89 -- "entirely or almost entirely" Iranians -- would be offered the choice of either pressing ahead with an asylum claim and being sent to PNG, or transfer to another country.

He defended a major Aus$2.1 million publicity blitz on the new scheme targeting key migrant groups from Afghanistan, Pakistan, Iraq, Iran, Sri Lanka and Vietnam, as hundreds of protesters gathered for snap rallies across Australia.

In contrast, Foreign Minister Bob Carr said he'd been "struck by the level of support both in the Pacific region and beyond" for the plan.