Objectors to the proposed Garden Bridge addressed Lambeth councillors on Wednesday night and urged them not to allow council land on the South Bank to be used for the river crossing.
Supporters of the Thames Central Open Spaces (TCOS) campaign want Lambeth Council to thwart plans for Thomas Heatherwick's controversial pedestrian bridge by refusing to allow council-owned land on the South Bank to be used for the bridge's southern landing.
The affected land on the South Bank is owned by the council and leased to Coin Street Community Builders. Before the Garden Bridge can be built, the council must agree to vary that lease.
Michael Ball, Hannah Quigley and Ernie Hearn from TCOS addressed Wednesday's full council meeting at Lambeth Town Hall to make their case.
"We want to talk about an issue of principle: the sale of public green open space for commercial gain," said Michael Ball.
"This sale will allow development of a huge building of 6,500 sq ft on top of our open space.
"Most of that building is entirely superfluous to the bridge: two thirds of it will be a commercial building with cafes, shops and restaurants."
He added that the proposed deal would be "incredibly disadvantageous to Lambeth's threadbare coffers".
Hannah Quigley told councillors: "This is Osborne and Boris's vanity project. Why are you proposing to subsidise it with Lambeth's precious green open space?"
Cllr Jack Hopkins, Lambeth's cabinet member for jobs and growth, insisted that no decision to allow the council land to be used for the bridge project had been made.
"I can assure you that myself and the Labour administration here will make sure – if that should happen – that every condition will be looked at to protect the space, to make sure that we get value ... and that the value goes back into the community," he said.
"We are not at that stage yet."
It emerged at Wednesday night's meeting that Lambeth Labour councillors are coming under pressure from local party members to block the Garden Bridge, with the general committee of Vauxhall Constituency Labour Party recently passing a motion asking the council to prevent the bridge being built.