These dams are estimated to produce 6000 MW and 3750 MW respectively but on completion would inundate a massive area in Siang belt, SIFF President Tasik Pangkam said.
The consent of the affected land owners was not taken before MoUs were signed with the power companies, Pangkam said.
The state government had signed the MoUs without carrying out any proper survey or social assessment, he claimed.
Meanwhile, former Chief Minister and BJP leader Gegong Apang, also opposed the large dams proposed in the belt and announced he would join the agitators.
"If huge fund could be spent on cleaning the holy Ganga, some respect should also be shown equally holy to Siang river and the Brahmaputra," Apang said.