E Manoj, a local office-bearer of RSS, was waylaid on a road by the gang which threw a country-made bomb at him and then hacked him to death before fleeing the area near Thalassery, police said.
RSS-BJP leaders alleged the CPI(M) was behind the killing and gave a call for a dawn-to-dusk hartal in the state tomorrow.
BJP President Amit Shah, who was in the state capital, condemned the killing.
But tension resurfaced with the BJP and CPI(M) poaching workers from each side in recent times.
Last year, a large number of BJP workers led by a prominent local leader had joined the CPI(M).
In retaliation, the saffron party lured scores of CPI(M) workers to its camp last month.