Blockade Australia has begun a "Sydney mobilisation to resist climate destruction", with groups disrupting intersections in the city.
Television footage on Monday showed a group of about 50 masked protesters marching north down Kent Street towards the harbour after moving through the city, beginning at Elizabeth Street, between Park and Bathurst streets.
NSW Police are out in force in Sydney's CBD and have a heavy presence on the Harbour Bridge after Blockade Australia threatened a resurgence in their campaign which has previously shut down Port Botany, the Harbour Bridge and Spit bridge.
The Transport Management Centre says the Sydney Harbour Tunnel is closed at the tunnel entrance in North Sydney, after a protester parked a car blocking access.
Blockade Australia livestreamed a video of the young woman in her car blocking the tunnel claiming she was from Lismore, the epicentre of unprecedented floods earlier this year in northern NSW.
All traffic is being diverted via the Sydney Harbour Bridge and traffic is backed up for sever a kilometres.
Blockade Australia said it would continue to cause disruptions in the days ahead.
"Disruption to the infrastructure of Australia's project of exploitation is essential in cutting through the climate denial that this system survives off," it said.
Earlier this year, the NSW government passed legislation to crackdown on disruptive protesters following several arrests of climate change activists blocking traffic and access to ports.
Protesters now face a maximum penalty of two years' jail and $22,000 fines for disrupting traffic or preventing access on roads.
The Roads and Crimes Legislation Amendment Bill also creates new offences targeting people blocking access to major facilities including ports and railways.
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