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 men and women, aged between 21 and 70, on the bridge just after 5am on Tuesday, NSW Police said.

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

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

Calling on the major federal political parties to declare a “climate emergency”, the hanging trio unfurled banners – emblazoned with “100% renewables” and “make coal history” – and bright red flags as they dangled beneath the bridge for several hours.

Officers from NSW Police, with help from PolAir, marine and rescue crews eventually removed and arrested the remaining six activists by 9.30am.

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.

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Earlier, Greenpeace Australia Pacific chief executive David Ritter brushed off questions about the legality of the protest action, describing the activity as “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.”

Mr Ritter said Australia faced a climate emergency.

“We’re here today to say ‘no more’,” he said.

The protest comes four days before the federal election.

Campaigning in Adelaide on Tuesday, Mr Morrison said he was “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 in Tasmania.

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

-with AAP