The police arrested five CPI(M) workers, including two local committee secretaries, in connection with the attack on a Kerala State Electricity Board (KSEB) office at Padur, near Alathur, on Tuesday.
The attack had been on those who reported for duty ignoring the trade union call for strike on Tuesday.
Those arrested were CPI(M) Padur local committee secretary Pramod Peechamkottil, 41; Kavassery local committee secretary Rajaneesh Kalarikkal, 35; Prasad Maruthampadam, 37; Radhakrishnan Thavakkal, 54; and Anoop Kunnumpuram, 28.