Eye witnessess said the monk poured petrol over his body and then lit himself. He was rushed to the hospital with severe injuries, the police added.
The monk had been calling for a stop to cattle slaughter in Kandy, the city home to one of the holiest shrines for Buddhists, the Temple of Tooth, which contains relic of the tooth of Buddha.
The incident marred the Wesak festival celebrations, an important day among the island nation's Buddhist majority population.