Accompanied by Bihar BJP president Mangal Pandey and other leaders, Modi began his fast outside the Sri Krishna Memorial hall near Gandhi Maidan police station.
Speaking on the occasion, he lambasted the state government for police firing in Bagaha in which six Tharu tribal people were killed last month.
He also accused the government of adopting "insensitive attitude" towards the victims by failing to provide Rs ten lakh compensation to kins of victim as demanded by the BJP.
Had the state government paid heed to the intelligence tip off and enhanced security at the Mahabodhi temple and adjoining areas, yesterday's serials blasts in the temple town could have been prevented, he said.