Unidentified persons hurled country-made bombs at two local libraries run by Congress at Peralassery and Thannada in Kannur on Sunday, the Kerala police said.
Some furniture was damaged and windowpanes were shattered, they said.
Police said they were yet to identify the miscreants and no arrests have been made so far.
However, local Congress activists alleged that CPI(M) was behind the attacks.
KPCC president V M Sudheeran condemned the attack.
The attack comes amidst an ongoing tussle between the CPI(M) and the Congress over attack on a Dalit leader and his daughters by members of the Left.