Papua New Guinea authorities have entered the decommissioned Manus Island detention centre and given some 400 men still in the facility one hour to leave, according to multiple sources.
There are unconfirmed reports authorities have been aggressive, and one detainee claimed two men had been "beaten up", but no casualties had been reported.
As of 9am, refugees said PNG naval officers were patrolling outside the camp but had not moved inside.
The early morning action follows repeated deadlines issued - but not enforced - by the PNG government over the three weeks since the centre was shut down.
"Immigration and police started searching the rooms and are saying, 'Move move, you only have an hour to move'," Iranian-Kurdish refugee and journalist Behrouz Boochani said.
"They suddenly came in at 6.50 [this] morning. The police, special forces, police squad are hundreds and are spreading in the prison camp and around the prison.
"We are in high alert condition right now. We are under attack."
PNG police, who yesterday said the relocation of the remaining refugees had been "deferred" until a Supreme Court appeal was heard, could not confirm an operation was underway.
Police spokesman Dominic Kakas said: "There was supposed to be one [an operation] yesterday but it was suspended because of the Supreme Court appeal."
Ben Lomai, the lawyer lodging the appeal, told Fairfax Media the court had on Wednesday agreed to a full bench hearing on December 15.
About 400 men remain in the centre and have been refusing to relocate to three new "transit centres", citing fears for their safety, the adequacy of the new facilities, and their desire for permanent resettlement in a third country.
The apparent spike in tensions three weeks after the facility was shut down came as former Australians of the Year penned an open letter to Malcolm Turnbull and Bill Shorten urging them to address the "human disaster" unfolding on the island.
In recent days, shocking new footage and testimony from inside the centre has shown filthy blocked toilets, makeshift rainwater collection and men consuming 20 to 30 cups of coffee a day in an attempt to replicate the effects of their anti-depressant medication.
More to come
