Four people have scaled a sail of the Sydney Opera House to protest what they say is a "humanitarian crisis" on Manus Island.
The Whisteblower Activist and Citizens Alliance says the climb began just after 10am to highlight "the inhumane treatment on Manus Island of men seeking asylum".
"The Opera House is an iconic symbol of Australian culture and we think Australia is becoming increasingly internationally recognised for its human rights abuse to refugees," WACA spokeswoman Lily Matchett told AAP.
"Australia has become a world leader in cruelty - cruelty endorsed by both Liberal and Labor governments."
Police vans and a helicopter are at the scene on Thursday morning.
On Manus Island, almost 600 men have barricaded themselves inside the Papua New Guinea island's mothballed detention centre, a week after it was shut down.
They believe it's safer to remain in the compound than risk being attacked by locals at new facilities near the main township of Lorengau.
