A crude bomb was hurled at a Bharatiya Janata Party office in the heart of the city in Thiruvananthapuram, with the party alleging that Communist Party of India-Marxist workers were behind the attack.
No one was injured in the incident, police said.
The incident occurred around on Tuesday night, shortly after BJP state president Kummanam Rajasekharan had left the office for Kozhikode, where preparations are on for the three-day national executive and council meeting from September 23 which is being attended by senior party leaders and Prime Minister Narendra Modi, party sources said.
At least four workers were on the top floor of the building, housing the office, when the bomb was hurled.
Though no one was injured, glass panes of the main entrance door were damaged in the incident, said Thiruvananthapuram City Police Commissioner S Sparjan Kumar.
The incident comes close on the heels of a series of sporadic clashes between CPI-M and BJP workers in the past few months in northern Kannur district.
Rajasekharan alleged that the CPI-M, the lead partner of the ruling Left Democratic Front in Kerala, had taken law into its hands and police was a "mere spectator."
"Violence of the CPI-M is increasing each day. A BJP activist was hacked to death in Kannur recently and the CPI-M is behind it," he told reporters at Kozhikode.
"Despite so many incidents, CPI-M leaders and Chief Minister Pinarayi Vijayan, who is also holding the home portfolio, have not condemned the series of attacks against the BJP and its workers," he said.
BJP leaders P K Krishnadas and M T Ramesh slammed the CPI-M, saying the bomb attack against the party office should not be seen as an "isolated" incident.
"This is a calculated and well planned attack against the BJP and CPI-M is behind it," they said. All flex boards in front of the office were also destroyed, Krishnadas said.
Television channels telecast a CCTV video footage showing a man riding a motorcycle minutes before the blast and police are on the lookout for him.
Meanwhile, the CPI-M said the BJP's allegations against it were "baseless" and it had no connection with the incident.
BJP's allegations are malafide, and people will reject them, the party said.
"The incident is highly condemnable. Anti-social elements connected with the incident should be immediately arrested", CPI-M Thiruvananthapuram district secretary, Anavoor Nagappan, said in a statement.
The probe into the incident should be led by top level police officers, he said.