Four abseilers are dangling from the Sydney Convention Centre as part of an environmental protest outside BHP Billiton's annual general meeting.
Police riot squad and rescue officers were called to Darling Harbour on Thursday, where about 40 people have gathered to protest against the mining giant.
Police said several groups were involved in the protest, including anti-uranium protesters and Aboriginal elders.
Four people abseiled down the building on Thursday to unfurl two banners reading "BHP, Dirty Deeds" and "Australian uranium fuelled Fukushima".
Police at the scene told AAP they expect to charge the four people who had scaled the building.
Protest organisers said the abseilers originally intended to climb down unaided but after learning they were likely to be charged decided to remain on the side of the building.
Arabunna man Kevin Buzzacott was among environmentalists who gathered at the convention centre forecourt for the launch of an alternative annual report, Dirty Deed.
The report, the fourth of its kind, focuses on the uranium and copper mine at Olympic Dam, South Australia, and the company's proposed liquefied national gas hub at James Price Point, in Western Australia.