Hundreds of protesters called for a sewage works not to be built near their homes.
More than 300 Ovingdean residents, brandishing banners and placards, chanted at passing cars travelling along Marine Drive on Saturday.
An hour earlier, a separate demonstration saw 200 residents from 12 community groups converge in East Brighton Park, Whitehawk.
A site in Ovingdean and two sites in Sheepcote Valley, Whitehawk, are being considered by Southern Water as the location of a new sewage treatment plant to serve Brighton and Hove.
Southern Water has drawn up a shortlist of eight sites and has asked the public to vote on their preferred location. The public consultation is due to end today.
The Ovingdean protest was organised by the Campaign for Ovingdean Against Sewage Treatment.
Group chairman Robert Mojab said: "It is magnificent to see practically all of Ovingdean's small community joining together to protest against these proposals.
"A sewage treatment works should not be built on a green-field site but on a brown-field site as the Government recommends.
"It is ludicrous for Southern Water to think it is environmentally sound to build it here, on a site which is proposed to be included in the boundary of the South Downs National Park."
At East Brighton Park, which forms part of Sheepcote Valley, residents formed a huge human circle, while chanting and singing.
Scotland-born Terry Hall, 72, from Henley Road, Whitehawk, played the bagpipes.
Whitehawk and Manor Farm Community Association chairman John Commons said: "As small splinter groups, we were not able to get very far, so we came together to form one action group to protect East Brighton Park.
"We refuse to stand by and watch our beautiful park, which is used by so many people, be destroyed.
"East Brighton is a deprived area and Southern Water want to take away the one amenity which is free for all of us here to enjoy. "
Southern Water will make a planning application after considering the views of the public.
A spokeswoman said: "The project team will look to choose the preferred site on which to make a planning application. This will be based on many factors, including proximity to the existing pumping stations, planning restrictions, environmental matters and sustainability."
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