Protesters who broke through barriers to walk across Auckland Harbour Bridge are threatening a repeat that would make yesterday's demonstration look like a "walk in the park".

Up to 2000 people pushed through bushes and jumped fences to get on to the bridge, forcing police and transport authorities to close the northbound lanes for an hour.

Tens of thousands of motorists had Sunday plans disrupted, and a tanker carrying urgently needed oxygen to North Shore Hospital was stuck, until police organised a passage through.

The protesters want a walkway and cycle crossing on the bridge - something the Transport Agency has said will not happen.

GetAcross spokesman Bevan Woodward, who organised the protest, warned the agency that it should reconsider its position.

"If the Transport Agency doesn't pull its head out of the sand and wake up to the high level of support of this and how vital it is to the Auckland region, then there will be another protest and it will be a whole lot bigger than today," he said.

"It will make today look like a walk in the park."

The 2000 walkers and cyclists gathered near the Curran St on-ramp to the harbour bridge at 9am yesterday for a peaceful protest.

They called on the Transport Agency's regional director, Wayne McDonald, who was flanked by police officers, to grant them passage to "their" bridge.

But he remained unmoved by polite pleas, and then jeers of "no-brain Wayne".

Mr Woodward had earlier urged protesters not to use force to get on to the motorway saying, "We will be insistent, but we will not and we must not force our way over."

But when a couple of protesters got onto the bridge, others quickly followed.

Twenty-three-year-old Erin Allison-Maxwell ran in front of the northbound traffic holding his hand out in a stop sign, as others pedalled and strode past police and motorists towards Northcote.

Mr Woodward said the thousands of protesters who did force their way across demonstrated a "sense of people power, the sense of frustration people have".

Police made no arrests, but Waitemata road policing manager Superintendent John Kelly said they were disappointed that the protesters broke their word.