However no casualties were reported.
The bomb exploded about 50 metres from Chakarakkal police station, police said.
"It is likely that the station was the target as there are no other buildings on the road leading to it, they said, adding, the incident occurred at 4.15 am.
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BJP sources said the crude bomb exploded hardly 45 minutes after state President Kummanam Rajasekharan left the office.
To protest over the incident, BJP is taking out a march to the Secretariat here at 10 am, the sources said.