Blockade Australia says it has begun a "Sydney mobilisation to resist climate destruction", with groups disrupting intersections in the city on Monday morning.
Protesters converged "to cause disruption to the political and economic systems of Australia which are driving climate destruction".
A group of about 50 protesters marched north through several streets after meeting at Hyde Park about 8am.
The group dispersed when it was blocked by police at the intersection of George and Bridge streets, blocking access to the Harbour Bridge.
NSW Police are out in force in the CBD and have a heavy presence on the Harbour Bridge after Blockade Australia last week flagged a resurgence in their activities, which previously shut down Port Botany, the Harbour Bridge and the Spit bridge.
The Transport Management Centre said at 8.30am the Harbour Tunnel was closed at the North Sydney entrance after a protester parked a car, blocking access.
Blockade Australia live-streamed a video of the young woman in her car, saying she was from Lismore, the epicentre of unprecedented floods this year in northern NSW.
All traffic was diverted via the Harbour Bridge and traffic was backed up for several kilometres.Â
The tunnel was reopened after the car was removed, the Transport management Centre said at 9.12am.
Blockade Australia said it would continue to cause disruptions all week.
"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.
Olivia, a spokeswoman for the group, said Sydney had been where Australia's destruction of the continent had been most intense.
"Our collective survival rests on organised opposition and the use of strategic direct action to stand against this project of destruction."
Earlier this year, the NSW government passed legislation to crackdown on illegal protesters following several arrests of climate change activists blocking traffic and access to ports.
Protesters face a maximum penalty of two years' jail and $22,000 fines for disrupting traffic or preventing access on roads.
The legislation also created new offences targeting people blocking access to major facilities such as ports and railways.
Blockade Australia said it would hold a media conference on Monday afternoon.
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