They said 34-year-old Rajesh's left hand was chopped off in the attack around at 9 pm.
Police said an investigation has been launched and the search is on for the assailants.
BJP state president Kumanam Rajasekharan alleged that the CPI-M was behind the attack, a charge denied by the district leadership of the left party.
Police are maintaining a strict vigil and clamped prohibitory orders for three days from yesterday after BJP's Kerala unit office in Thiruvananthapuram was vandalised and the house of CPI-M state secretary Kodiyeri Balakrishnan's son Bineesh Kodiyeri was attacked.
Earlier, 10 people, including four from the student and youth wings of CPI-M, had been arrested in connection with the violence in the city yesterday.