Opposition to building flats in country parks is gaining momentum, with almost 10,000 people in four days liking a Facebook site that opposes the idea.
More than 100 people, meanwhile, have said they will join a protest hike in the Tai Lam Country Park on Sunday.
Former Observatory director Lam Chiu-ying set up the Facebook group "Hong Kong Country Parks are my breathing space" last Friday.
"I've received many beautiful country park pictures taken by those who liked my group, including one of a starry night above the park," said Lam, who has described the idea floated this month by development chief Paul Chan Mo-po as a cancer cell.
On the page, Lam says he feels heartbroken that many people are eyeing the country parks as a source of housing land.
"You need to protect the parks if you still want to see a starry night. You can't buy that with money," he said, calling for like-minded people to "like" his page.
"One may say a starry night does not mean much. I would say a child can hardly be broad-minded if he or she has never seen a starry night."
Lam, now teaching at Chinese University, helped draft environmental policies for Leung's election manifesto.
He spoke out strongly against the idea of occupying some of the parks with flats when Chan said on his blog on September 8 that the issue merited discussion.
"If you give away 100 square feet now, later you will ask for 100 square feet more. Ultimately, it will destroy the original aim of having country parks … to enjoy nature," he said.
An alliance of 18 green groups named "Save Our Country Parks" is organising Sunday's hike, which has attracted more than 100 people since it was advertised on Facebook two days ago.
Alliance member Paul Zimmerman, chief executive of Designing Hong Kong, said he hoped the hike and other activities, including a public forum next month, would also bring public attention to unprotected enclaves inside country parks.
He said the enclaves, left outside the park boundaries and owned by villagers, were threatened by development.
A recent example was the government's plan to allocate 2.5 hectares of the 8.45 hectare enclave in Hoi Ha, next to the Sai Kung West Country Park, for village-type development. The area can accommodate more than 100 three-storey village houses.
"The area will be damaged once the homes and roads are built," Zimmerman said.