A Students' Federation of India (SFI) leader was attacked allegedly by BJP workers at Parassala in the district, police said on Monday.
The SFI leader sustained serious injuries in the late Sunday night attack, they added.
Two Bharatiya Janata Party (BJP) activists were arrested in connection with the incident on Monday, the police said.
Four activists of the ruling Communist Party of India (Marxist) in Kerala, including the SFI leader, and three BJP workers have been hospitalised after political violence erupted at Parassala, the border area of Thiruvananthapuram district.
Abu Thahir, a district committee member of the SFI, was hacked, leading to head injuries.
The SFI is the students' wing of the CPI(M).
"Santhosh Kumar (36) and Girikumar (38) are the two who have been arrested in the case. They will be produced before a court on Monday. Further investigation is on," a police official told PTI.
Houses and vehicles belonging to both CPI(M) and BJP leaders have come under attacks since yesterday.
Local sources said two houses of CPI(M) leaders were attacked and in retaliation, two houses of BJP leaders were vandalised.