Save Radley Lakes campaigners staged a protest on Saturday in the battle against RWE npower's plans to dump fuel ash into Thrupp Lake.
About 500 supporters gathered at Abingdon Guildhall at lunchtime to voice their objections to the power firm's plan.
A dozen speakers, including Abingdon mayor Peter Green and local MP Dr Evan Harris, addressed the crowd outside the hall.
Several others stepped up to the microphone to pledge support, including Andrew Boddington, the national campaign manager of the Council for the Protection of Rural England.
Pupils from Radley Primary School provided support by holding up individual letter cards spelling out slogans such as "Switch off npower", which were cheered.
Dr Harris said: "It's a pleasure to be here, but also a sadness to have to fight to save this important element of our local environment."
He claimed npower had chosen the "cheapest option" of dumping fuel ash, which he described as "an environmental disgrace".
Dr Harris also attacked the power company's premature clearing of woodland at the site before the determination of the current Town Green application, which will be the subject of a public inquiry next month.
On Friday, campaigners welcomed a decision to halt work after a bird's nest was discovered at the site.
RWE npower said the development, which will eventually take ash from Didcot Power Station, would be delayed until late summer. The firm is adamant its work is necessary and has taken on an independent ecologist to monitor the land.
But Mr Green said: "When something is wrong - and this is wrong - you've got to stand up and be counted to stop this.
"It's for our kids, and their kids."
The majority of the protesters then took part in a two-mile trek along the cycle route from Abingdon to Thrupp Lake, where they were joined by cyclists who had ridden from Oxford.
Mr Crowley said: "It's important that we keep up the pressure on RWE npower to stop plans that will ruin Thrupp Lake.
"Dumping ash containing toxic substances will pose a persistent long-term threat to the local environment.
"It will inflict immense damage on a treasured Oxfordshire beauty spot and its wildlife."
RWE npower was not available to comment.