Police said the CPI(M) worker Kannan and the bjp workers had had a bet during the state assembly elections last year.
Kannan had stated that the saffron party would not win a single seat in Kerala and that he would shave off his moustache if the result was otherwise.
Former Union minister O Rajagopal had won the lone seat for the BJP in the polls.
Yesterday, the four workers caught hold of Kannan, held his arms behind and forcibly shaved off half his moustache, police said.