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Climate protesters arrested in Sydney

A woman who chained herself to the steering wheel of her car and blocked the Harbour Tunnel is one of 11 climate protesters arrested in Sydney.

The protesters were among a group of 50 to 60 activists who kicked off a week of disruption in Sydney with a march through the city centre on Monday.

Blockade Australia began its threatened protests "to cause disruption to the political and economic systems of Australia which are driving climate destruction", with a group converging at about 8am at Hyde Park.

They marched towards the harbour while chanting, playing drums, pulling down signs, dragging wheelie bins onto the road and blocking intersections.

The group were met with a large police presence, including officers on horseback and in helicopters overhead.

Protesters dispersed when police stopped them at the intersection of George and Bridge streets, blocking access to the Harbour Bridge.

Aerial footage from the Nine Network showed a car pushing through a group of protesters at a CBD intersection.

Assistant Commissioner Paul Dunstan said police were investigating the incident but blamed protesters for their "unlawful" acts of disruption.

"The behaviour of this group was nothing short of criminal activity," he told reporters.

"The throwing of bicycles, the throwing of garbage bins, the throwing of other items in the path of police, in the path of media, in the path of innocent members of the public just walking by, will not be tolerated and cannot be by the people of NSW," he said.

Blockade Australia live-streamed video of a 22-year-old woman who parked a car at the northern entrance to the Harbour Tunnel.

The tunnel was shut down about 8.15am when she blocked all city-bound lanes for about an hour with traffic backed up for several kilometres.

NSW Police says she put a bicycle lock around her neck and tethered herself to the steering wheel before being removed from the car and arrested.

The group said the woman was from Lismore, the epicentre of unprecedented floods this year in northern NSW.

Police were out in force in the CBD and had a heavy presence on the Harbour Bridge after the group last week flagged a resurgence in their activities, which previously shut down Port Botany, the Harbour Bridge and Spit Bridge.

Mr Dunstan noted the haphazard nature of Blockade Australia's tactics proved to be troublesome for police, before arrests were made.

"The group this morning was highly unorganised and erratic, and they were moving throughout the CBD in an unstructured format. It was difficult to get ahead of them," he said.

Blockade Australia said it would continue to cause disruptions all week but did not specify the type of action.

"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.

Sally-Ann Brown, an organiser with the group, defended the necessity of the small and scattered demonstration saying it was needed because policymakers were not listening.

She urged ordinary citizens to participate in the environmental call-out.

"The emergency room on this planet will not be big enough if the temperature continues ... Change happens through radical action ... Join us," she told reporters on Monday.

Earlier this year, the NSW government passed legislation to crack down on illegal protesters who now 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.