Protesters scale Sydney Harbour Bridge

Climate change activists have abseiled down the Sydney Harbour Bridge holding protest banners, with at least three reportedly arrested.

Six Greenpeace activists scaled the bridge at dawn this morning to demand that Prime Minister Scott Morrison takes action on climate change.

Three of the protesters have unfurled banners demanding action on climate change after they abseiled from underneath the pedestrian walkway on the western side of the bridge.

One activist can be seen holding a banner that says "100% RENEWABLES", with the Greenpeace logo underneath.

A NSW Police spokesman said an operation was under way after a "number of people were detected" on the bridge.

It remains unclear how long the protesters will remain on the bridge.

A spokesperson for the Transport Management Centre told news.com.au Lane 8 on the Harbour Bridge was momentarily closed around 6am this morning, but has since reopened.

He said some ferries were affected by protesters abseiling beneath the bridge, but that the overall affect on morning commuters would be "minimal".

The activist group posted a link to a petition targeting Mr Morrison just before 6am this morning.

"Dear Prime Minister Scott Morrison, Australia's climate is in a state of crisis," it says. "I call on you to declare a climate emergency now. I stand with the #ClimateEmergency activists and I am willing to do what it takes to ensure the next government acts on the climate crisis, including taking part in the growing movement of peaceful defiance."

"Climate damage is happening right now. Australia is facing a climate emergency right now," Greenpeace Australia Pacific CEO David Ritter said in a statement.

"Our political leaders must listen to those already affected by climate disaster and act."

He told reporters this morning that the protests are a "wake-up call to the Prime Minister that there is a climate emergency".

More to come...