Suriyasai Katasila, coordinator of the Green Politics Group, has condemned the gun attack on rubber protesters, killing one guard and seriously wounding another early on Sunday.
He called for authorities concerned to quickly find the culprits, to prevent the situation from getting deterioated.
The attack had ruined a chance, which was already slim, for further negotiations between the protesters and the government.
The Yingluck Shinawatra government was also to blame or allowing the protest at two sites in Cha-uat district of Nakhon Si Thammarat province to drag on for 10 ten days without showing any serious intention of ending it, Mr Suriyasai said.
The Green Politics Group coordinator said the government believed the protest was political and had therefore used state mechanisms to drive a wedge among protest leaders in the local and organisational levels.
Moreover, it had instigated the people's hatred towards the protesters, causing them to feel being isolated by the government.
While the people were in deep trouble, the prime minister was busy with her scheduled overseas trips and never thought of going to see the protesters by herself, Mr Suriyasai said.