The group has won the support of the Victorian Society and the Garden History Society (GHS) - two organisations that would not normally involve themselves with such a building.
Landowner the London Development Agency (LDA) is currently working with the proposal, for which it has widespread local support, of demolishing the London County Council architects' 1963 building.
But opposition to the bulldozers is now growing. Victorian Society spokesperson Kathryn Ferry said her group would refuse to support the centre's demolition.
'The Victorian Society believes that imaginative plans for the country's most important municipal park are long overdue and we would encourage the LDA to take a holistic approach to this historic landscape,' she said.
The GHS is also lining up alongside C20 to save the building. GHS Principal conservation officer Jonathan Lovie said: 'The park today is depressing and barren, and the C20 Society proposals offer a well-balanced way forward.'
The society has also won the unlikely support of design consultant and Channel Four presenter Naomi Cleaver.
She said: 'The Crystal Palace Sports Centre is Grade II* listed for a reason, and that's because it's a majestic, grand building that has served the community magnificently.
'Just because it is in need of refurbishment, do we really need to knock it down and start again? Surely this is throwing the baby out with the bath water, not to mention even more costly.'