Several people, including 18 police personnel, were injured in the stone pelting and lathicharge, police said.
The activists were protesting the alleged discharge of effluents into the Chalakudy river by the plant, a joint venture between Japanese firm Nitta Gelatin Inc and Kerala State Industrial Development Corporation.
Trouble arose when police stopped the activists of NITA Action Council, spearheading the protest, when they set out on the march to break the pipelines carrying the waste.
Police resorted to lathicharge to disperse the protesters.
The injured were admitted to hospitals at Chalakdy, Angamaly and Thrissur, police said but did not specify the number of the wounded protesters.
The Action Council called for a strike in the district tomorrow to protest against the police action.