Climate change activists have scaled Sydney Harbour Bridge, with at least three reportedly arrested.

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

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

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

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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."

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, which could cause traffic chaos for peak-hour commuters this morning.

More to come...