Man, 70, among 15 arrested over Harbour Bridge climate change protest

A 70-year-old man is among 15 environmental activists who have been charged after the group allegedly blocked security and protested on the Sydney Harbour Bridge to demand action on climate change.

Security staff spotted the male and female activists, aged between 21 and 70, on the bridge just after 5am today.

The group allegedly blocked security to allow six members to lower themselves from the public walkway and under the iconic structure.

Nine of the activists were arrested on the walkway while three managed to get under the structure and three abseiled from the bridge.

"The actions of the six Greenpeace supporters today is a wake-up call to Australia that there is a climate emergency," Greenpeace Australia Pacific chief executive David Ritter told 9News.

The Greenpeace supporters unfurled banners calling for action as they hung from positions underneath a public walkway on the western side of the bridge.

The signs read “100% renewables” and “make coal history”.

Supporters also took over a public walkway on the bridge.

Live Traffic Sydney said one lane of traffic was closed about 6.30am due to a "police operation" but it re-opened minutes later.

NSW Police removed protesters from beneath the bridge with the help of officers from the Rescue and Bomb Disposal Unit, Marine Area Command and PolAir.

All 15 were charged with risking the safety of another by climbing a structure, wilfully preventing free passage of a person/vessel, climbing on any part of a bridge and entering enclosed land without lawful excuse.

They have all been granted conditional bail to appear at Downing Centre Local Court on June 5.

Mr Ritter brushed off questions about the legality of the protest action, describing the activity "like a smoke alarm when the house is on fire".

"It's a wake up call to the prime minister who is not acting in the face of this climate emergency," he said.

"He still has time before polling day to make a statement."

The protest comes four days before the federal election.

Prime Minister Scott Morrison today said that he's "passionate about a lot of things but I don't want to stop traffic on the Sydney Harbour Bridge because that would be inconsiderate."

Opposition leader Bill Shorten also disapproved of the illegal protest.

"What I say to millions of Australians who are concerned about climate change - you can make a really effective protest on Saturday," he said.

"You can vote Labor for re-election on climate change."