Protesters tied light-blue ribbons around the gates of the government headquarters at Tamar in another attempt to save Lung Mei near Tai Po from being turned into an artificial beach.
But the government reiterated yesterday it will go ahead with the project, planned for completion in 2015.
"Light-blue symbolizes our vision to protect the ocean," said Dickson Wong Chi-chun, organizer of Alliance on Protecting Lung Mei.
Wong threatened to apply for a judicial review if the government does not withdraw its approval of the beach project before Friday. "The government was not aware the site has such a high ecological significance before it approved the plan," he said.
Chanting slogans and waving banners, the protesters urged the government to save about 200 endangered marine species, including sea snails and seahorses found near the proposed artificial beach site.
Organizers said 3,000 protesters turned up but police put the number at 370.
A protester, who has lived in Lung Mei for 11 years, said: "Lung Mei is not an ideal site for a beach. Its water quality is poor. Residents normally swim at Ting Kok East Beach just a few minutes away from Lung Mei."
Another Tai Po resident, Celine Ma Tai-lo, criticized the government solution of relocating the endangered species to Ting Kok East Beach.
"It is not practical," she said. "Humans have living habits and so do sea creatures. How could the creatures adapt to a new environment in such a forceful way?"
Booths sold postcards of the marine creatures found in Lung Mei, with funds going to help the campaign.
A government spokesman said Tai Po District Council endorsed a motion on Thursday supporting the government Ting Kok Coastal Conservation Plan and requesting swift implementation of the Lung Mei project.
"The project will be rolled out as planned," he said.
In another development, Executive Council member and Heung Yee Kuk vice chairman Cheung Hok-ming admitted he has land in Tai Po but denies it will benefit from the beach project directly. He says his support for the plan has little to do with the property.