The activist, identified as Roma Malik and her aide Sukalo were arrested from the former's house in the main market in Robertsganj Kotwali police station, Inspector K D Yadav said.
Yadav said police was looking for here since April after her name came to limelight during the agitation against the Kanhar project here.
"The head worker of National Forum for Forest Peoples and Workers was arrested on charges of instigating tribals to encroach forest land and asking those displaced because of Kanhar project to launch an agitation against the government and creating hurdles in the project," police said.