The People Network for Sustainable Development, which includes members of top environmentalist groups nationwide, gathered in front of Government House on Wednesday.
More than 150 police officers are reportedly guarding the protest site at the Public Sector Development Commission Office, which is across the road from the Government House.
A representative of the group, Lertsak Khamkongsak, read a statement on Wednesday morning proclaiming that the people have a right according to the Constitution to demonstrate peacefully, so the group was using this right to protest against the new Enhancement and Conservation of National Environmental Quality Bill.
The bill has been criticised by activists saying that not only did it not solve problems with existing environmental and health impact assessment procedures, it will intensify those problems, and will make it even easier for problematic and harmful projects to get approval.
The statement also pointed out that the new bill has been pushed forward very rapidly to the National Legislative Assembly (NLA) without taking heed to public-sector suggestions about its flaws.
Lertsak stressed that the protesters would not return home unless the government takes satisfactory action over the bill.
Prime Minister Office deputy secretary-general Sompas Nilapund met the protesters and received letters to take to the government in the afternoon.
Meanwhile, the bill is under the consideration of NLA. It was approved on its first reading on November 24.